Morris tunes - a big collection of tunes from the English countryside (sorted by village!)
Richard Robinson's tunebook - another giant collection from all over the world. He's over in the UK, so the server's a bit slow, but it's worth the wait, especially if you need gif format, but he's got ABC format, too.
The Fiddler's Companion at Ceolas
Tunes at Ceolas - nice Celtic music archive plus many other types of resources, including descriptions/explanations of the instruments, styles, dances, artists, and even sound samples!
ABC2win software - a nice little program that converts the gibberish of ABC music notation into the gibberish of standard music notation with the option of playing the tune for you (in case you learned by ear) (mirror site)Mudcat Cafe folk song database - this is an immense database of songs without sheet music, but they have guitar chords and midi links so you can hear the song, and if you want to learn something to sing when you don't have your violin with you, ...
Fiddle fork - place where you can exchange fiddle tunes