Wordorigins.org

Cassidy, Frederic C., and Joan Houston Hall, eds. Dictionary of American Regional English. Four vols. Cambridge, Mass.: Belknap Press, 1985-2002. Volume 1: Introduction and A-C, Volume 2: D-H, Volume 3: I-O, Volume 4: P-Sk

One of the most ambitious linguistic projects 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. The fifth and final volume is due out in 2011.