What's your recording setup? What gear do you have already?
Are you looking for something with built in samples that you can record the line output analog audio from,
or something that will trigger a virtual instrument/sample library like SSD, Addictive Drums, or drumcore?
Singular Sound BeatBuddy is pretty slick and has some good features. You do need a computer to manage songs/patterns and there is a bit of a learning curve with that. Besides the ability to play pattern based songs with fills and transitions, users have also created complete songs which include other instruments such as bass and piano.