Writing and the Internet
English 249S-01: Advanced Expository Writing
Merriam-Webster Dictionary On Line
Jack Lynch's Guide to Grammar and Style
Strunk and White's The Elements of Style. A classic (but a bit dated?).
A Web of On-Line Dictionaries. This site not only gives you access to English words and definitions but to many other languages.
Dr. Watson. The HTML equivalent of a spell and grammar check, and it's free, unlike some other sites of this type, you are free to use this site's coding checker on your projects.