Apple released a version of its GarageBand music creation software for the iPad back in March, and now it has converted that into a universal version that runs on iPhone and iPod Touch as well. As with the original iPad version, you don't get the same range of features as in the original Mac product, but you can export tracks created on your device into the desktop version for further refinement.
This is more likely to be a productivity killer than an enhancement, but I suspect lots of iPhone owners will be racing to download it. GarageBand costs $5.49, and requires iOS 4.3 or later. Existing owners of the iPad version can upgrade for free to the new combined version.