AmpliTube on the iPhone and iPad are both great. You may be surprised that you can do more than just jam on the go with them, as much fun as that is. They also have a built in multitracker you can add on, 4 tracks on iPhone, and 8 tracks on iPad. Each channel has reverb, compression, and EQ, too.
You can import backing tracks to jam to or for mixing in any available track. You can then apply any of the stomps, amps, or rack effects to the tracks, mix, and pan.
GrooveMaker is the perfect companion for creating backing tracks for AmpliTube. You can create a beat and export it at 16/44.1, then import it directly into AmpliTube. And you can always do all of your producing using loops in GrooveMaker, if guitars aren't your thing.