Check out this application from Technimo LLC, iReal b has been created for OS X and iOS devices as a practice tool which you can use to learn songs, practice improvising over chord changes or practicing exercises over a backing that is a bit more interesting than a Metronome. Check out the demo video above, it looks pretty cool.
iReal b can be used as a book: Create, edit, print, share and collect chord charts of your favorite songs for practicing and performing. Thousands of chord charts are available for free on the iReal b forum. iReal b can be used as a virtual band: Practice with realistic sounding piano, bass and drum accompaniments.
Check out more here – http://www.irealb.com/