Don't overlook midi files as a great source of tunes. There certainly aren't as many as ABC on the web, but with Noteworthy (or any commercial sequencing software) you can print the notation.

For example, here is a great site that someone in a newsgroup shared with us recently. It's Mal dnd Jo's Home Page. They offer a selection of Irish tunes already in sets. Really great stuff! 'Very kind of them to share these tunes with us. There's enough material here to keep the party rockin' for hours!