The Elements of Style - Strunk and White

The classic reference text for writers:

http://www.bartleby.com/141/index.html

Oxford Dictionary

Pretty straight-forward - put in a word and get back a definition:

http://dictionary.oed.com/entrance.dtl

The Free Dictionary

Medical, Legal, and Computer dictionaries along with standard English reference tools:

http://www.thefreedictionary.com/

Merriam-Webster

A few changing topics featured daily, your standard dictionary and thesaurus plus Word of the Day, word games and more:

http://www.merriam-webster.com/

Roget's Thesaurus

The classic thesaurus we all know and love:

http://www.bartleby.com/110/

Bedford/St. Martin

Writing exercises in grammar, style, punctuation and mechanics from a leading academic publisher:

http://bcs.bedfordstmartins.com/exercisecentral/