Chiptune lovers and composers, here's something for you. SiON140 and MMLTalks are two software synthesizers which use the Music Macro Language (MML) to represent musical notes and synthesizing information, both already having a decent library of user contributions that could be listened to or shared with others.

Between the two, MMLTalks has more features while SiON140 was especially designed so that short MML compositions be shared via Twitter. Under the SiON140 interface, there's a small play button at the start of each comment that you can click on to play any user submission instantly. For MMLTalks, all you have to do is click on the song name and the tune will play automatically. The only thing you need to run either application is Flash Player 10, although composing something that's as masterful as some of the song contributions will require spending time studying the SiOPM MML reference. More information here and here.