Rhythm Studio very neatly bundles a step sequencer, mixer, and a variety of sound and rhythm generators into one ridiculously low‑priced app, and also provides virtual interconnectivity and a fair amount of touch‑screen control.
Check out Arturia's take on this Oberheim classic dual-VCO synthesizer module with oscillator sync, pulse-width modulation and two-pole multi-mode filter.
Mira is the first incursion into the iPad app world from Cycling '74, makers of the popular Max graphical software toolkit for audio and media applications on Mac and PC. iTrigger is a grid-oriented MIDI controller app from POJ Studios.
Apple's iPad seems to have wormed its way into both live and recorded music in a big way, so having some way to hold the thing securely in the right position is a must...