General

• HowStuffWorks:

  A guide to the workings of things, from the mundane to the complex

• IPL: The Internet Public Library, sponsored by the University of Michigan, provides links to educational sites in the arts, sciences, and other fields.

Anatomy

• Bartleby: The unabridged, classic Gray's Anatomy of the Human Body includes nearly 1,300 etchings of body parts and systems

Archaeology

• CyberPursuits: User-friendly index compiled by a Texas Web designer

Astronomy

• Skymaps.com: Free monthly sky maps

Biology

• Department of Biochemistry & Molecular Biophysics: An overview of major topics

Chemistry

• WebElements: A guide to the periodic table

Dictionary

• Merriam-Webster: A free dictionary/thesaurus from Merriam-Webster

Math

• Wyzant, Inc.

• Education 4 Kids, Inc.

• Martindale’s

Psychology

• Psychology.org

Writing

• bartleby: William Strunk Jr.'s classic Elements of Style (1918 edition)