If you're looking for a programmable app that emulates the classic Roland TR-808 and 909 drum machines, and the TB-303 bass synth, you'll love TechnoBox (V. 1.9, AudioRealism; $9.99). It does a good job of emulation and has a deep programming interface.

FEATURES, AT A GLANCE

303 Emulation based on the ABL2 engine
Drum module with selectable 808 or 909 sounds
Shuffle with variable amount for drum module
Transpose and randomizer for synth module
24 patterns per session
Pattern sequencer with up to 1,000 bars
Session manager with real-time switching
Onboard quick-start manual
Supports multitouch for knobs

Find out more information about TechnoBox.