Purdue Online Writing Lab

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

Subjects:

Writing
English as a second language
Job search and professional writing
Research
Style and grammar

Features:

Exercises
Guides
Articles
Teacher and tutor resources

Guide to Grammar and Writing

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

Bunches of information on grammar, writing, and style.

Subjects:
Grammar
Vocabulary
Writing
 - Composition
 - Research papers and essays
 - Paragraphs
Misc
 - Writer's block
 - Avoiding plagarism
 - Editing and proofreading writing

Features:
Interactive quizzes
Articles
Question help
FAQ

Life Write

http://www.lifewrite.com/html/class.htm

Free online writing course by an author.

Features:
Assignments
Articles