Ask Oxford
The site contains guides to spelling and grammar, letter writing, writing in plain English, communicating effectively, and text message emoticons and acronyms. To top it all, you can even download sample letters for free.
http://www.askoxford.com/

Economist.com - Style Guide
Tips on writing clearly and concisely.
http://www.economist.com/research/styleguide/

Guide to Grammar and Writing
Easy-to-use guide to English grammar and writing.
http://ccc.commnet.edu/grammar/

Jack Lynch's Guide to Grammar and Style
Online guide to grammar and style that also contains many useful links.
http://newark.rutgers.edu/~jlynch/Writing/

Jennifer's Language Page
Foreign language dictionaries, grammar guides and other such resources.
http://www.elite.net/~runner/jennifers/language.htm

The Blue Book of Grammar & Punctuation
Use this site to find answers to your questions concerning proper English grammar and punctuation.
http://www.grammarbook.com/

William Strunk Jr - The Elements of Style
Outlines the principal rules of writing in plain English, and highlights the most commonly broken rules of usage and composition of the language.
http://www.bartleby.com/141/