Theres a few sweet apps ive run across. One simple thing, Gstrings is a tuner that is accurate, nice ui, and will tune whatever you need. Another, if youve ever used guitar pro for tabs or sheet music. Tabdroid is basically the perfect mobile version of this and even uses the same guitar pro files. Its a couple bucks but works amazingly well. Robotic guitarist has a free version, gives you a tuner, metronome, and some other goodies, but still could use some work. Theres quite a few synthesizing/beat making programs too, that have really open possibilities, even rivaling the apps i had on my old iphone.
I had downloaded a program that would come in handy, that claimed to record in a non compressed way to record better quality files. Which would be priceless for music jamming. But the app wouldnt work, but i hope to see something like it again soon.