The Fiddler's Companion is Andrew Kuntz's online descriptive index of traditional dance tunes commonly found in the repertoires of players from the British Isles and North America. It contains abc files for a great many airs and dance tunes, as well as references to printed sources. It has its flaws and there are mistakes, but it is an exceptional resource.
Now it has been improved. The new and improved Fiddler's Companion, dubbed the Traditional Tune Archive, is fully searchable, enabling users to interact with it in ways not possible through the old interface. Use the links in the frame on the left side of its homepage to explore TTA's new features. "Drill down" to really dig into what you're looking for, whether it is a dance type, a composer, or a regional origin. Cross reference different search results to hone your search. The site is still under construction (Fiddler's Companion is immense and the migration is time consuming) but I recommend bookmarking TTA and checking back frequently.
You can also bookmark my site -- both links have been posted in the links section right here on Gumshoe Arcana.