Here is a list of what is on mine:
Jammit (haven't really tried it yet)
Virtuoso (probably will delete...don't think it is useful in learning real piano)
Korg iMS-20 - software recreation of the analog synth - seriously cool app
Ampkit - free version - really haven't used it much but heard that full version is better than AmpliTube
AmpliTube - amp modeler on your iPad; you'll need an iRig or Peavey's (ostensibly better) solution; DO NOT EXPECT AN AXE ;-)
Xenon Groove Synthesizer - cool little multi instrument studio. My only gripe is that the sequencer does not seem to be set up to handle weird time signatures, of which I am fond
TabToolKit or Guitar Pro - I have both; haven't really settled on a favorite
Korg iELECTRIBE - software version of the ELECTRIBE sound box
iREALb - the REAL Book. The problem is that you don't get the head melody; just the chord sequences. And, I think buyers after a certain version didn't even get much of the chord changes, because of legal action. :-(
Guitarist Ref HD - get your chord and scale dictionary, reversed chord finder, note finder, etc. This one works quite nicely. My gripe is that the author of this app has the perfect platform to do a "Guy's Grids" sort of thing to show the evolution of chords, but hasn't done it.
LickoftheDay - looks cool, but wants you to buy stuff pretty quickly. From Guitar World.
AniMoog - recommended earlier by another poster; we have used it in recording projects
TheMusicPath - lessons for various instruments. Some are free; some you have to buy. Production looks good, but I have not yet worked through any of these.
Moog Filtratron - VCO and VCF with a variety of options for sound generation - very cool