I use Rhythm Rascal but the freeware limits you to .wav only unfortunately. It does let you save and if you have a prog that can convert files then you'd be set.
Rhythm Rascal itself is dead easy to use and really lets you arrange the beats you make into a song very easily.
I use Hydrogen with the colombo acoustic drum kit to slap up some drums, export them to wav, and then import the wav into an audacity project for recording/mixing.