GUIDE TO GRAMMAR AND WRITING: A rich set of tools from the Capital Community College Foundation in Hartford, CT.
THE PURDUE ONLINE WRITING LAB: Resources for students and writing teachers.
11 RULES: Short, sweet, valuable. Many other good tools and tips on the home page.
JACK LYNCH'S GUIDE TO GRAMMAR AND STYLE: A concise and useful guide to grammar rules and usage.
WRITING.COM: One of the oldest web-based writing sites (established in 2000).