Fun new app from Apple:
Music Memos automatically analyzes the basic arrangement and suggests chords you played for each track. Then you can hear how your music sounds with a realistic virtual drummer and bass player jamming along. If your performance speeds up or slows down, so does the band. You can even tweak the energy and performance of the drums or bass with a few simple controls.
I played around with my guitar and Music Memos a bit today at lunch and it's a blast. I only wish that there was some kind of a visual onscreen metronome to help keep me from rushing or dragging.