Howard Levy is the presiding genius on the diatonic harmonica, the guy who used overblows to explode our idea of what is possible. Rarely does one individual musician have this sort of effect on an instrument; shock waves from his accomplishment will continue to be felt for decades. His album HARMONICA JAZZ nailed me to the wall back in 1987, with its fully-chromatic version of "Donna Lee" in bebop time, but he's kept on growing, expanding, and shows no signs of resting on his laurels.
P.T. Gazell is a harp player's harp player who plays country, swing, and pop instrumentals with full chromaticity on a valved diatonic harp. Check his stuff out!
Jason Ricci is a harp player that all contemporary harp players surely know about by now; if you don't know about him, please drop what you're doing and check out his stuff: original, soulful, blazing fast, overblow-driven 21st Century blues harmonica:
Billy Gibson, Mississippi-born, is THE harp player on Memphis's renowned Beale Street these days, and he's got the voice, the chops, and the band to back it up:
Dennis Gruenling isn't just a superb and original player who has made the lowwwwww harps his own; he's a great teacher, in part because he was the most intensely focused student of the harp I've ever seen. If you still haven't heard his first album, "Jump Time," buy it:
Harp-L is the THE internet discussion list for all things harmonica. Ask any question and you'll get a half-dozen (or three dozen) well-informed opinions:
SPAH (Society for the Preservation and Advancement of the Harmonica) is THE organization for all things harmonica. Their annual conventions are the stuff of legend; pretty much everybody shows up and throws down at the evening concerts and jam sessions:
I first encountered Annie Raines, harp master, in 1989 at the Chicago Blues Festival, where she was sitting in with some local guys under the trees behind the stages in Grant Park. She was great then and she's masterful now. Check out her blog on Road Food:
If you're looking for a neat little harp mic--an alternative to the big, bulky Astatic and Green Bullet--you'll definitely want to take a look at the Bottle o' Blues mic:
Rod Jones is a painter and harp player (R&B Jones) from the UK who offers, among other things, insightful commentary on harmonica teachers. Spend a little time at his quirky, one-of-a-kind site:
Justin Sandercoe is a blues/jazz guitar-god over at YouTube; his philosophy of teaching--give all you've got, and then some--is one I heartily endorse: