Common Errors in English (Paul Brian)
http://www.wsu.edu/~brians/errors/errors.html

Commonly Confused Words
http://www.pnl.gov/ag/usage/confuse.html#n

Common Mistakes and Tricky Choices (English Plus Grammar Slammer)
http://englishplus.com/grammar/00000160.htm

NPR Online Style Guide (Marcus D. Rosenbaum and John Dinges)
http://www.npr.org/inside/styleguide/pug.htm

The Writing Center: Writing Guidelines (Tidewater Community College)
http://www.tc.cc.va.us/writcent/handouts/writing/index.htm

Commonly Misused Words (Edmonds Community College)
http://web.edcc.edu/gvb/misused.html

The Elements of Style (William Struck, Jr.)
http://www.bartleby.com/141/index.html

The Hypertext Book Twenty Common Usage Errors (Daniel Kies, College of DuPage)
http://papyr.com/hypertextbooks/engl_101/errors.htm

Common Errors (Serendipity)
http://serendipity.magnet.ch/errors.html

The American Heritage Book of English Usage (Bartleby.com)
http://www.bartleby.com/64

Usage Quiz (over 100 questions)
http://hobbes.ucsd.edu/quiz.html