Virtual Training Suite

http://www.vtstutorials.co.uk/
A large set of free tutorials geared towards helping college students research more efficently in their area of study. Written by various lecturers and librarians from universities. Tutorials take about an hour to finish.

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