Sampulator - Make (and record) beats on your keyboard
ReSlice - Slice your audio with an advanced, rhythmic arpeggiator and FX.
Ableton Note - A playable iOS app for forming musical ideas
nlog Synth - Professional Virtual Analogue Synthesizer ::: check nlogmusic at YouTube :::
Groovebox: Beat & Synth Studio - Make Music & Play Instruments
cassini - Polyphonic Synthesizer for iPhone/iPod touch