This sampling of links is meant to be broadly representative of folk sites out there. Most of them provide their own list of resources and links. Have fun here, then go play some tunes!
Ceolas – the quintessential Celtic music site, hosted out of Stanford University. Start here and you may end up anywhere in a few days of browsing, that is, if you come up for air. Good site for exchanging and converting on-line music – text, midi, etc.
The Session – I think the best tune exchange for traditional tunes. Based upon the tune writing method of abc, it’s a great crowd-sharing resource for maybe 30,000 tunes. Discussion, where to find recordings of the tune, midi, abc, image of the written version of the tune.