JamBuddy will turn your iPhone or iPod Touch into… well, a jam buddy. It will provide either mandolin or guitar accompaniment for 50 familiar jam standards, plus you can program in as many other songs as you like. Playback speed is adjustable and songs can be entered in either 3/4 or 4/4 time.
As the JamBuddy page in iTunes tells it..
JamBuddy will keep playing all night and never complain that you are hogging the spotlight.
The app is compatible with any iPhone or iPod Touch running iPhone 2.2.1 software.