JC's ABC Tune Finder:Database of tunes; downloadable in a variety of formats, including abc, PNG, GIF, PDF and MIDI.
Contemplations from the Marianas Trench: Folk music of Britain, Ireland and America with lyrics, midi files, historical notes and background for traditional tunes. Includes the Child ballads, sea shanties and Turlough O'Carolan's music.
Henrik Norbeck's ABC Tunes: Indexed source of ABCs for traditional Irish and Scandinavian tunes, organized by title, song type and key.
Jack Campin's Homepage: A freelance researcher and musicologist maintains a considerable resource of traditional music (in abc format) mostly taken from old and rare sources otherwise difficult to access.
Richard Robinson's Tunebook: A collection of traditional and new tunes in traditional styles, including Irish, Scandinavian, English, French, Balkan and others. Available as ABC source, and as small black-and-white gif images.