Apple updated GarageBand with the launch of iOS 5.1 and brought in new features that addressed the one major shortcoming of the previous version - the ability to edit your work. There are a host of other new features as well, including track merging and a new Smart Strings instrument.
It costs £2.99 and is a real bargain considering it is by far the most fully-featured music-making and editing app for iOS available. If we do have a complaint though - only a minor one, mind - it's that support for an external keyboard or microphone is only available on the iPad version of the app.
Essentially a suite of sequencers, ReBirth for iOS impressed us with its ease of use and the quality of sounds and effects available. It costs £4.99 for the iPhone/iPod touch version and £10.49 for the iPad and it is integrated with Facebook, meaning you can work on collaborations with friends.