Golden Ring/New Golden Ring from Folk Legacy. There is also a great 3-CD set put out by (I think it's Smithsonian Folkways) that has a sampling off all kinds of American music including old-time fiddle, various blues types, early country, gospel, Appalachian, etc, and lots of liner notes. I've got an old record called "Folk Music if the Southern Appalachians" that is very good, and was recently rereleased on CD.
Of course, it depends on what interests you. In the US you might look for Jean Ritchie, Hazel Dickens (Appalachian), Etta Baker and Elizabeth Cotten (Piedmont blues), Jimmie Rodgers, the Carter Family (AP, Sara, Maybelle--early country), Joseph & Cleoma Falcon (Cajun), Hoyt Ming, Fiddlin' John Carson, Woody Guthrie (of course), and Bob and Evelyne Beers.
From the 1960's-1980's revival, the early recordings of the Swallowtail Contradance Band, the Red Clay Ramblers, Jay Ungar (if you can find any early records), and the High Woods String Band are all great. I'm mostly into old-time fiddle music so my "associations" are somewhat limited, but the Red Mountain White Trash, Vulcan's Britches, Freight Hoppers (now defunct, but with two good CD's available), Volo Bogtrotters, Dwight Lamb.