This is a wiki (a website that anyone can edit) with sheet music for various songs and other musical pieces.
The sheet music is written in abc, a musical notation language.

There are different sections:

• traditional music can be viewed and edited by everyone
• copyrighted music can only be viewed by a select group of people
• music on personal user pages cannot be viewed by anyone except the registered user who added it

The wiki uses DokuWiki and the ABC2 Plugin.
You can use the abc syntax when it's surrounded by <abc> and </abc>.
You can transpose each piece by adding the number of semitones, e.g. <abc 7 -5>.
The rest uses wiki syntax.

You can try everything out in the playground.