Rhythm generators and players

Native Instruments Battery 3

Apart from not having an internal drum pattern generator/sequencer, Battery 3 has almost everything you need in a drum sample player. It comes with a huge drum sound library, and several nifty features for "humanizing" drum parts that threaten to become too repetitive and robot-like. (In particular, you always have to watch out for the dreaded "machine gun" effect, where repeated strikes of the same drum does NOT sound like repeated strikes of the same drum, but rather like a machine gun.)

DK+ Virtual Drums

Sometimes, however, you need something machine-like, and then the built-in rhythms and pattern sequencer of DK+ Virtual Drums comes in very handy. Although it can perform quite lively and varied renditions of real drums, I mainly use it as a vintage electronic beat box, letting it play samples of old Roland drum machines... stuff like that.

Jamstix

A drum machine with a difference. It is rule-based rather than pattern-based, which means that given general guidelines, it comes up with its own rhythms and variations. It can also jam to audio or MIDI input (hence the name), varying the volume and intensity of its playing according to your playing.