Wikipedia (the ultimate in encyclopedias)

http://en.wikipedia.org

The Writing Lab at Purdue University (a premiere source for all kinds of online writing help)

http://owl.english.purdue.edu/owl/

The Writing Center of Capital College (features a bank of common grammar questions & answers)

http://grammar.ccc.commnet.edu/grammar/

Junket Studies Tutoring (with a list of 11 simple writing rules)

http://www.junketstudies.com/rulesofw/

Writing Center of Amherst College (help for writers who feel stuck)

http://www.amherst.edu/~writing/stuck.html

Study Habits (tutorials and study guides for writers)

http://www.study-habits.com/