Open source Linux drum machine Hydrogen
has made its way to Mac OS X; Johnathan Dempsey has created binaries
for easy installation (10.3 required). Hydrogen is still in
development, but this is worth trying, and you can bet you'll see many
more Linux audio tools on the Mac. Install Jack
first if you haven't already, and have fun with drum-pattern editing,
sampled drums, and Linux LADSPA plugin effects on your Mac!

Even better: send suggestions and bugs to the Hydrogen developer list.

[Updated -- direct links]
Download Mac OS X binaries
Download source code