Milkytracker (
http://www.milkytracker.net/ ) is a sample-based sequencer in the flavor of oldschool trackers like FT2, IT, and so on. It would make a pretty good drum sequencer, if you don't mind finding a few drum samples to stick in there
It's got some bugs, and the slide-out keyboard gives it fits, but it does run well most of the time. And of course it's free