Though nobody ever called him a "folkie" and lived to tell about it, I'd have to throw in a vote for Bill Monroe. The influence of bluegrass music upon the larger folk music scene has been huge. Listen to a fair sampling of current US-based folk musicians who aren't considered bluegrass acts and you'll still hear a lot of bluegrass going on. Bluegrass has also served as a point of entry for many musicians who have ultimately wound up playing other styles. Most of the folks I know who play Irish or old-time music, myself included, cut their teeth on bluegrass. And we won't even talk about guys like Bela Fleck and David Grisman 'cause they ain't folk...