Linguistic Atlas Project: This website contains data on ten linguistic atlases of different regions of the United States. Some of the data is available for download.
Personalized Dialect Map This quiz, based on the Harvard Dialect Survey, tells you where your personal dialect is located on a map. (It basically tells you how likely people from a certain area are to respond the way that you do to the survey questions.)
General surveys
Varieties of English (Kortmann and Upton 2008): A collection of articles covering the phonology, morphology and syntax of varieties of English around the world, in four volumes and an interactive CD-ROM.
Appalachian English: A gorgeous site hosted by the University of South Carolina devoted to Appalachian English, with tons of resources, links, and information.
MultiMo: A database of multiple modals, with lots of data, references, and other information.
Multilingual Manchester: A nice comparison of the English spoken in Manchester with other parts of the U.K., including maps.
Language On Trial: Rachel Jeantel: A discussion of the language of Rachel Jeantel’s testimony in the Trayvon Martin case, including an interview with Stanford University linguist John Rickford.
The delights and frustrations of off-road grammar: An article by Neal Whitman on theweek.com discussing phenomena where the grammar of English doesn’t seem to give speakers an easy way of expressing what they’d like to say. Discusses gradable adjectives, embedded reciprocals, and double passives.