This is the culmination of Rick Aschmann's years-long "hobby" of collecting dialects. It's a comprehensive and detailed map of the dialects (and sub-dialects!) of English-speakers in Canada and the United States.
While the work of many years, the map was updated on April 17, 2003, and surged in popularity shortly thereafter. This map is cropped due to size, but the full sized one is below
Aschmann's site is a veritable font of information on English dialects. There's the Dialect Information Chart which tells you which vowel sounds can be found in what dialect and each dialect's "unique features." Like Mat-Su Valley Alaska, which has the unique feature of being "strongly like North Central" but with some "main Alaska dialect" mixed in. If that doesn't mean anything to you, there's a helpful parenthetical there: "See Sarah Palin."
Aschmann bases his map and dialect information on the Atlas of North American English, his own research created the names of some of the dialects and made adjustments to their borders. The one on his site is especially useful because clicking on a place takes you to audio sample of that dialect on his curated list of audio examples of many of the dialects. This is great because a) reading a description isn't nearly as good as hearing it and b) it's not a random sample, but the one he's picked as the best.
How does this strike you? Did you know some people don't pronounce caught and cot the same? What dialect is yours? If it's one Aschmann needs a sample of, why not help him out?