the Chicago Manual of Style    
http://www.chicagomanualofstyle.org/contents.html

the AP Stylebook
http://www.apstylebook.com/

Garner's Modern American Usage

Common Errors in English Usage
http://www.wsu.edu/~brians/errors/errors.html

Grammarphobia.com
http://www.grammarphobia.com/

Dictionary.com
http://dictionary.reference.com/translate/

Merriam-Webster Online
http://www.merriam-webster.com/

Macmillan Online Dictionary
http://www.macmillandictionary.com/

the Oxford English Dictionary