DrumTrack8 for the iphone or ipod touch is pretty neat. You can programme basic beats and you get a bunch of different kit sounds - playback is via the iphone or the ipod. The other one i use on a PC (but it's pretty basic looking by todays standards... think of a big spreadsheet) is Drumsite, which will let you programme beats and songs. The free one won't let you store the track you've built, but the paid for one does... and then you output the track to a wav or mp3 and playback using an mp3 player.