the abc musical notation language
abc is a language designed to notate tunes in an ascii format. It was designed primarily for folk and traditional tunes of Western European origin (such as English, Irish and Scottish) which can be written on one stave in standard classical notation. However, it is extendible to many other types of music and recently Steve Allen has coded Beethoven's Symphony No. 7, Movement 2 in abc! Since its introduction at the end of 1991 it has become very popular and there now exist several PC and UNIX based tools which can read abc notation and either process it into staff notation or play it through the speakers of a computer.
One of the most important aims of abc notation, and perhaps one that distinguishes it from most, if not all, computer-readable musical languages is that it can be easily read by humans. In other words, with a little practice, it is possible to play a tune directly from the abc notation without having to process and print it out. Even if this isn't of interest, the resulting clarity of the notation makes it fairly easy to notate tunes. In addition, the ability to write music in abc notation means that it can be easily and portably stored or transported electronically hence enabling the discussion and dissemination of music via email.

How to interpret abc music notation
an introduction to, and comprehensive tutorial on, the abc music notation system.
A tutorial by Steve Mansfield

Resource pages
The ABCyclopedia

Svenska abc collections  
Henrik Norbeck's ABC tunes
Frank Nordberg's abc site (Musica Viva)
Lennart Sohlman's Swedish Fiddle Tunes

Folksång från hela världen
abc tunes from Cranford Publications
Alan Ng's transcriptions
Bernard Loffet's abc's and diatonic accordion tablature
Bernie Stocks' Irish Music Books
Chris Falt's fiddle tunes
Dan Beimborn's collection of Irish tunes
Dan Cobb's Irish Tunes
Folk, Midi und abc-Format
Henry Ford's tunes: Northumbrian Minstrelsy and Walsh's Lancashire
Howard Mitchell's tunes
Irish tunes for the Anglo Concertina
Liam Hart's Gaelic Song Texts
Loretta Reardon's European and American fiddle tunes
Malcom Downie's Tune Sets
Richard Robinson's Tunebook (plus other abc files)
Robert Blakeley's Irish Music
Ronan Keryell's Breton tunes
Seymour Shlien's Balkan & Israeli Dances
Simon Wascher's Tune Collection
Stephen Klein's fiddle tunes
Steve Allen's Morris library
Steve Mansfield's abc tunes from Le Grand Session de Manchester
Sylvain Piron's French Trad abc collection
The Gaelic Song archive
The North American Fiddlers Association's tune collection
The Tunes (O'Neil)
The Tune Book
Trish O'Neil's tunes from the web
Tune Search in ABC format: This site has tunes in ABC format, no sounds.
Tunes at Ceolas