Dictionary of American Regional English

One of the most useful things any research program in any field can do is provide a resource to other researchers. The Dictionary of American Regional English is a rich linguistic resource decades in the making, and it is now available online. 

Here's a description from the project's About page:

The Dictionary of American Regional English (DARE) is a multi-volume reference work that documents words, phrases, and pronunciations that vary from one place to another place across the United States. 

Challenging the popular notion that our language has been "homogenized" by the media and our mobile population, DARE demonstrates that there are many thousands of differences that characterize the dialect regions of the U.S. 

DARE is based on face-to-face interviews carried out in all 50 states between 1965 and 1970 and on a comprehensive collection of written materials (diaries, letters, novels, histories, biographies, newspapers, government documents, etc.) that cover our history from the colonial period to the present. 

The entries in DARE include regional pronunciations, variant forms, some etymologies, and regional and social distributions of the words and phrases.
A striking feature of DARE is its inclusion in the text of the Dictionary of selected maps that show where words were found in the 1,002 communities investigated during the fieldwork.