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:
https://en.oxforddictionaries.com/
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:
Bedford/St. Martin
Writing exercises in grammar, style, punctuation and mechanics from a leading academic publisher: