Bleep is a new chiptune drum app for iPhone. This is by the same guy who makes the 8-Bit Studio app.
He actually makes a ton of apps, none of which meet the high standards we've come to expect from $0.99 apps. This looks like it might have some sound design options though, which would justify the huge price tag. It even has MIDI input!
Here is a short demo of the wavetable in this drum machine.