Drumcircle

Drumcircle is a loop sequencer with a variety of built-in samples and a very clean interface. It can export to .wav files, has 8 save slots for later editing, and features an online collaboration mode where all users can work together (or against each other) to create music.

The user interface is like a retro painting program combined with a radar sensor. Like a painting program, there are six palettes of seven different colors or shapes that can be dragged and dropped on the screen, in this case representing audio tones and effects. Like a radar sensor, Drumcircle plays the samples as the rotating beat hand passes through them. And everything's so pixellated (intentionally) that it's undeniably retro.

A very cool feature of this program is the online collaboration mode. This mode downloads a tune that has been created by other users of the program, which you are then free to edit (for better or worse, of course). It's a quick and dirty way to crowdsource some inspiration if your own fiddlings don't get you far enough!