IK Multimedia just released their latest version of amp and effects software for iPhone users, AmpliTube 2.
The new software includes new features, some of which are free and part of the application and others available via in-app purchase.
Some things you can do for free are one-track recording, re-amping ability, export of recordings and mixes via email (as an MP3), and SpeedTrainer which allows you to slow down or speed up songs without affecting the pitch.
AmpliTube LE costs $2.99 and includes : 5 stompboxes, 1 amp and cab cabinet, and 2 microphones.
AmpliTube 2 is available for $19.99 and includes : 11 stompboxes, 5 amps and cabinets, and 2 microphones.