Amidio creates surprisingly powerful music apps for the iPhone--I've been particularly impressed with Touch DJ, a two-track mixer with effects and beat-matching, and Star Guitar, which is useful for guitar-based songwriters to sketch ideas when they're away from their instrument.
You control the melodies by moving your thumbs around two glowing discs on the main interface. Each disc has spaces for four notes. You can move your thumbs on the screen, or tilt and shake the entire phone, to control volume and vibrato, slide between notes, and switch between two different virtual instrument sounds. Each instrument in a particular scene is pre-programmed with a particular eight-note scale, but a configuration screen lets you change those notes, as well as control aspects like reverb and delay.
Each scene also comes with four discrete rhythms, which you can switch among by hitting buttons on the screen. The rhythms are perhaps the most sophisticated part of the app--each contains eight tracks, and you can choose from hundreds of drum and sequencer samples to create your own beats.
My only complaint with the app is that there's no way to add instruments to a scene on the fly--with only two instruments in any given scene, the sounds can grow stale after a few minutes. The app lets you keep a beat going while you reprogram the eight notes that each instrument can play, but it would have been nice if it let you switch instruments altogether.
Nonetheless, this app is a lot of fun for aspiring DJs and could be a fun way to keep people moving between live sets. Like Touch DJ, it's supposed to work with the iPad as well (I don't have an iPad so haven't tested it), and I think that larger screen would be a lot easier to work with. It's a whopper of a download, at more than 200MB, but well worth the hard drive space and the $6.99 price.