Grammar Resources (Online)

The Elements of Style
Strunk's classic text, online, on how to write well.

The Tongue Untied: A Guide to Grammar, Punctuation, and Style
An online course in grammar, punctuation, and style hosted by the University of Oregon.

Common Errors in English
Paul Brians's online text of commonly misused words in English and how to use them properly.

Dave's ESL Cafe
Dave Sperling's superb site to help ESL students with grammar, punctuation, mechanics, and usage.