International Writng Centers' Association Writing Resources
A directory of online handouts on writing from writing centers across the country; organized by topic.

Documentation

Chicago Manual of Style

MLA Style Guide

APA Documentation

Grammar and Mechanics

Grammar Girl
If you have a question about writing, generally it's been answered here. And it's entertaining.

Strunk's Elements of Style
Rules of grammar and composition, commonly misused and misspelled words.

Online English Grammar
A comprehensive guide to English grammar rules from the U.K. This site is geared towards students of English as a second language.

Reference Materials