American Dialect Society - http://www.americandialect.org/
Scholarly and dictionary association that studies the English language with particular reference to North America.
Cooldictionary.com - http://www.cooldictionary.com/
A free Webster dictionary. Includes sound links for certain words and features a complete edition.
Dictionary of Nigerian Languages - http://www.nigeriancomputersociety.com/dictionary/
An English language dictionary that translates to several dialects in Nigeria including Igbo, Yoruba, Hausa and Pidgin.