Writing Center Booklet:

Everything You Always Wanted to Know about Grammar

Grammar Help

Blue Book of Grammar
Daily Grammar
Guide to Grammar and Writing
Guide to Grammar and Style (Jack Lynch)
Handouts at Grammar Bytes (handouts and quizzes)
HyperGrammar

Dictionaries and Thesauri

Dictionary.com
Oxford English Dictionary
Roget's II from Bartleby
Thesaurus.com

Style Guides

APA
Chicago Manual of Style
MLA
Research and Documentation by Field (Diana Hacker)
Turabian