Wordorigins.org

Dictionary of American Regional English; Volume 1: Introduction and A-C, Volume 2: D-H, Volume 3: I-O, Volume 4: P-Sk; Frederick G. Cassidy & Joan Houston Hall, chief editors; Belknap Press of Harvard University Press.

Perhaps the most ambitious linguistic project currently underway, DARE is an attempt to capture the regional slang and dialect of America. A superb resource, unfortunately it is a work in progress, and therefore incomplete.