Grammar
The Armchair Grammarian   Grammar and punctuation guides are in the left column.
Guide to Grammar and Writing   A drop-down menu index of grammar rules.
The Online Writing Lab (O.W.L.)   Features rules on grammar, punctuation, and spelling.
Bartleby's Writing Elements   Rules of usage, composition, commonly misused and misspelled words.

Vocabulary
Vocabulary University   "Vocabulary puzzles to enhance vocabulary mastery." WordInfo   Latin and Greek morphemes (prefixes and suffixes) that make up words in English.

Reading
The Online Books Page   Lists over 25,000 free books on the Web.

Writing & Research
How to Write an Essay: 10 Easy Steps   Learn the fundamentals of essay writing.
SpellCheckOnline.com Spell check your texts for free!

Research Paper Guides
A+ Research & Writing   For high school/college students who need to write a research paper with step-by-step instructions, information on searching the Internet for information and other online resources.
Paradigm Online Writing Assistant   For older students, this site helps you organize and revise your writing. Further, the site helps you through the steps of writing either a thesis, an informal, an exploratory, or an argumentative essay.

Science

General Science
General Science   Information on types of science, rocks, plants, and everything else.
Chem4Kids   Information on matter, atoms, elements, and everything that makes up life.
Experimental Science Projects   An introductory level guide to conducting and recording scientific experiments.

Earth Science
Earth Floor   Middle school earth science. This site explains plate tectonics, biomes, the earth's sphere, geologic time, rock and water cycles, as well as an earth puzzle!

Natural Disasters
Savage Earth   This site covers the Earth's crust, earthquakes, volcanoes and tsunamis. You can also send a question to an expert.

The Environment
EPA Student Center   Learn about a number of environmental factors; from the Environmental Protection Agency.
Fact Monster: Environment   What the environment is and how it is effected.

Space
Exploring the Planets   Astronomy descriptions from the National Air and Space Museum.
Astronomy and Space Exploration   Descriptions of stuff in the universe.
StarChild   A Learning Center for Young Astronomers. Site maintained by NASA.

Energy & Resources 
USGS Educational Resources   Minerals, energy, land, water, and biota; as described by the U.S. Geological Survey. Above link is for grades K-6, for a link to grades 7-12, click HERE. 
Energy   An encylopedia-style listing of important energy terms.
EIA Kid's Page   It look like it's for little kids, but there's some really great info in there.

The Body
My Body   This site presents information about lungs, muscles, digestive system, teeth, skin, tongue, hair, heart and more.
Fact Monster: Body   Encyclopedia-style entries on parts of the body, body systems, and things that effect the body.

Science Fair Project Guide
Successful Science Fair Projects   Some tips for coming up with science fair project ideas, deciding how to turn an idea into a clever project, performing the science fair project, writing a meaningful report about it, and presenting a great-looking, sturdy display.

Social Studies/World History

Egypt
Egyptian Life   A description of daily life in ancient Egypt. Ancient Egypt   Site developed by the British Museum, quite of bit of information!
Ancient Egypt for Kids Facts, trivia, games, art, links for teachers and parents. Very extensive!

Greece
Greece   Greek art, its innovations, and the heros and gods the art depicted.
Daily Life in Ancient Greece   How people lived in ancient Sparta, Athens, Corinth, Argos or other Greek city-states.
Ancient Greece   A big resource list. A lot of topics to choose from.

Rome
Rome   Children's website from the BBC.
Daily Life in Ancient Rome   From Odyssey Online. 

Middle Ages 
Learner.org: Middle Ages   Descriptions of life in Europe during the Middle or Medieval Age. 
The Vikings   Site with video and interactive games from PBS.

Renaissance
Costume Super Center
Don't let the name of the webpage fool you. This is an extremely impressive directory of websites dealing with topics from the Renaissance.
Learner.org: Renaissance   A nice description of the intellectual and economic changes that occurred in Europe from the fourteenth through the sixteenth centuries.

Colonialism
Imperialism and the Balance of Power   Clicking on different continents gives you important information for that region during the Imperial Era.

Industrial Revolution
Tables Illustrating the Spread of Industrialization   The site contains statistics showing how industrialization spread in Europe from the 1700s to the 1900s.
Child Labor   The site by Spartacus School contains statistics showing how industrialization spread in Europe from the 1700s to the 1900s.

World War I
Summary of World War One.   A great site made by the BBC.

U.S. History

European Explorers
The European Voyages of Exploration   This site discusses European explorers and different areas of exploration.
Explorers of North America   This site focuses on the European exploration and colonization of North America.

Colonial America
13 Originals   Founding the American Colonies

Revolutionary War
The Timeline of the Revolution   Click on a listed event to see a brief description.
American Military History Online Some facts as well as links to other pages

Slavery
African Americans in Slavery   This site describes the slave experience in the United States.

Civil War
A Nation Divided   A timeline with links to important people and places.

Fine Arts

Art
Art Studio Chalkboard: Drawing   A resource for artists and art students that reviews the fundamentals and techniques of perspective, shading, color, painting and drawing.
The Artchive   An art history site that features biographies, theories, and criticisms of artists. Organized by artistic period.
Artcyclopedia   Find an artist’s galleries in no time with this easily navigable site.
Colors Matter   The concept of color approached from several disciplines: physiology, psychology, philosophy, and art.

Music
Essentials of Music   Composers, musical eras, and a glossary of terms.
Music Notes   Lessons on musical theory, history, styles, instruments, and a section on professions in music.

Photography
Focus On Photography   From the basics to composition and lighting. Also a section on the history of photography.
Photographytips.com   This site “is intended to help beginners get started in photography, and become so good at it that they turn into advanced amateurs.”

Theater/Drama
Learn Improv   A list of exercises that will help in learning improvisational comedy.

Foreign Languages

Visual Link Languages - Free lessons and activities for Spanish, French, Italian, Japanese, German, Chinese, Russian and English

Spanish
Basic Spanish   A beginner’s guide to grammar, vocabulary, and verbs. 
Spanish Language & Culture  (Formerly Spanish Grammar Exercises) A wonderful practice resource for all those fine tune grammar points that students struggle with so often.
Elemadrid   An automated Spanish verb conjugator. Claims to have 99% of all Spanish verbs in its database.
Spaleon   A Spanish verb conjugation trainer
Webspañol   This site promotes the study and appreciation of the Spanish language by utilizing a variety of Internet resources.
Study Spanish   Over 1,000 vocabulary words and drills for verb use.
BBC Languages: Spanish   For beginners, intermediates, and tutors. Learn and listen to improve. Gauge your level.

French
French Language Center   A collection of links put together by the City College of San Francisco.
French Online Grammar Quiz   Practice your French with these quizzes.
BBC Languages: French   For beginners, intermediates, and tutors. Learn and listen to improve. Gauge your level.

German
Canoo.net   An online resource for grammar, vocabulary, and pronunciation.
German for Travellers   This site gives tips on reading and vocabulary but also includes cultural information and German news and media.
BBC Languages: German   For beginners, intermediates, and tutors. Learn and listen to improve. Gauge your level.

ESL/ELL
Online Writing Lab: ESL Resources   Many different materials for English as a Second Language (ESL) students. Lots of links to other sites, too.
Sounds of English   Diagrams, descriptions, and audio practice exercises that will help you perfect your English speaking accent.
BBC: Learning English   English language material from BBC World Service.

Health

Food and Nutrition
All About Nutrition (And What's Best for You)   An explanation of protein, carbohydrates, and calories.
The Real Truth About Fast Foods And Nutrition   Allows you to compare the nutritional value of fast food or school lunch meals. Use the sidebar to navigate.
National Nutrient Database   A site made by the USDA which allows you to type in a food and get its nutritional information.

Drugs, Tobacco, Alcohol, and Caffeine
DEA Drug Descriptions   Descriptions of illicit drugs from the U.S. Drug Enforcement Agency.
NIDA for Teens   Fast facts on how different drugs affect the brain and body. 
NIDA Marijuana Information   More in-depth examination of marijuana. 
B.R.A.D. 21   (Be Responsible About Drinking, Inc) Gives facts about alcohol, its effect on the body, and discusses responsible drinking.
InfoFacts: Cigarettes and Other Nicotine Products Facts about tobacco use, by NIDA.
State Facts on Tobacco and Cancer   Numbers and percentages among tobacco users of different age groups

Sex Ed
Sexually Transmitted Infections   STD descriptions and prevention from Planned Parenthood.

Geography

Maps and Countries
Atlapedia Online   Full color physical maps, political maps as well as key facts and statistics on countries of the world.
CPL Databases: Countries   World regions, countries, photo gallery by world regions, recipes by country, famous people by country, data tables of info such as population and literacy rates, create your own comparison tables between countries.

Global Studies/World Regions
GeoHive: Global Statistics   A site with geopolitical data, statistics on the human population, Earth and more.
NationMaster   A massive central data source and a handy way to graphically compare nations economically and socially. 
C.I.A. World Factbook Information on all countries of the Earth. A good way to find quick facts.

Science

General Science
General Science   Information on types of science, rocks, plants, and everything else.
Chem4Kids   Information on matter, atoms, elements, and everything that makes up life.

Biology
Cells Alive   An explanation of a cell, its parts, and how it works
Science Stock Photography   See pictures of some stuff through a microscope.
The Genetics Source   An introduction to genetics.
Book Rags: Genetics   Alphabetical encyclopedia entries of genetics terms.

Physics
The Soundry   An explanation of the physics behind sound. 
The Physics Classroom   An online physics tutorial written for high school physics students.

Chemistry 
Chemistry Tutor   A general overview of chemistry. It's a good starting place.
Chem Team Index   An index of most aspects of chemistry.
The pH Factor   A further explanation of pH levels, acidity, and base.
Chemical Elements   An interactive periodic table with a lot of information on individual elements.
Buoyancy Basics   This page describes buoyancy in further detail, from PBS's Nova.

United States History

Colonial America
History of Jamestown   Also features a link to a timeline; by The Association for the Preservation of Virginia Antiquities
The People of Plymouth   A directory to individuals of note, places, and the important historical events of the Pilgrims (Separatists) of Plymouth.

Revolutionary War
Chronicle of the Revolution   These newspaper chronicles let you experience first-hand the events of the American Revolution as it happened; from PBS.
1777   Read about Washington's Continental Army during this important year. Included are battles, the capture of Philadelphia, and Valley Forge.
Timeline   A timeline from before the war through the signing of the Constitution. From the 1777 site.

Slavery
Spartacus Schoolnet: Slavery in America   A huge collection of people, events, laws, lifestyles, abolition, and personal accounts.

Civil War
Spartacus Schoolnet: U.S. Civil War   A huge collection of battles, timelines, photos, and people.
Civil War Biographies   Biographies of some of the important soldiers, politicians, and everyday people involved in the American Civil War.

Imperialism
The Age of Imperialism   An on-line history of the political and economic influence of the U.S. during the during the late nineteenth and early twentieth centuries.

World War I  
BBC: World War I  A huge collection of WWI material presented by the BBC. The site includes essays and galleries . Also, see the summary . . .                        Trenches on the Web   Selected tours of many aspects of the Great War.

World War II 
BBC: World War II   A huge collection of WWII material, including essays and galleries, presented by the BBC .
The Holocaust: A Learning Site for Students   A more in-depth look at the Holocaust, its causes, and its effects. From the US Holocaust Memorial Museum

Civil Rights 
Civil Rights History for Kids  A timeline of significant moments in the Civil Rights Movement.

Vietnam War
Vietnam Online   Timeline, biographies, maps and more. Presented by PBS. 
The History Place: Vietnam  A comprehensive account of Vietnam