On Writing and Editing:

The Chicago Manual of Style. 15th edition. University of Chicago Press
http://www.chicagomanualofstyle.org
A respected authority and the preferred style guide for publications in the social sciences. The website includes a question and answer forum that addresses issues not covered in the book.

Associated Press. Stylebook and Briefing on Media Law.
The style guide most often used in journalism and corporate communications. Recent editions include an Internet guide, sports guidelines, and business guidelines.

Amy Einsohn. The Copyeditor's Handbook.
An essential reference for any copyeditor. Comprehensive and accessible. Includes helpful sections on typecoding and on-screen editing.

Diana Hacker. A Writer's Reference.
Includes sections on formatting endnotes according to MLA, APA, and Chicago styles.

William Strunk and E. B. White. The Elements of Style.

American Association of University Presses
http://aaupnet.org

Association of American Publishers
http://www.publishers.org

On Linguistics:

Ancient Scripts.com
http://www.ancientscripts.com

Linguist List
http://www.linguistlist.org

On History and Current Events:

History Net
http://www.historynet.com

Economist.com
http://www.economist.com/index.html

Foreign Affairs
http://www.foreignaffairs.org

History Magazine
http://www.history-magazine.com

OneWorld.net
http://us.oneworld.net

World History Association
http://www.thewha.org

American Historical Association
http://historians.org/

For Fun:

Today in Literature
http://todayinliterature.com

Short Stories at East of the Web
http://www.short-stories.co.uk

Web del Sol
http://www.webdelsol.com

The Modern Word
http://www.themodernword.com

Literatura Argentina Contemporánea
http://www.literatura.org

El Mundo de Mafalda
http://mafalda.dreamers.com

American Copy Editors Society Online Quizzes
http://www.copydesk.org/quizzes.htm