What it says on the tin: javascript for rendering abc music notation. The abcjs libraries come in three basic flavors:

• abcjs basic: Call the library from javascript, passing the abc string to it.
• abcjs editor: transforms a textarea into an abc editor with scoresheet and audio.
• abcjs plugin: adds onto a webpage, rendering all abc it finds.

Using abcjs in your own websites is easy: InstallationDocumentation

Websites using abcjs:

• http://www.drawthedots.com (editor)
• http://www.tradzone.net/forum (plugin) (sample page)
• http://http://abcnotation.com/forums (plugin) (sample page)
• http://www.pmwiki.org/wiki/Cookbook/AbcTunebook (plugin and editor) (this is an addon for pmwiki for creating tune books)
• http://www.tunepal.org (basic)
• http://people.opera.com/howcome/2010/video/norway/ (Demoed during a keynote at Google I/O 2010 by Håkon Wium Lie, CTO of Opera)
• http://www.bestmusicteacher.com/ > left menu > music theory (Educational)
• http://www.eastofcleveland.com/flashcards/notereading.php (Educational)
• https://sightreadingfactory.com (Educational)

Some notes:

• ABC 1.6 is pretty much implemented (aside from some issues with slurs)
• Very simple multi-voice ABC and other features beyond ABC 1.6 are slowly making their way in, based on the behaviour of abcm2ps
• Midi playback is at a very early stage and does not yet feature ornamentation, dynamics, chords or multi voice.
• Midi playback will not work in Internet Explorer