Apple's iPhone already had software to turn it into a synth, sequencer and drum machine - now Sonoma Wire Works has released a four-track audio recording application.
Known, appropriately enough, as FourTrack, this lets you record tracks of unlimited length (the amount of memory you have left is the only restriction) and at 16-bit/44.1kHz quality.
There are meters that enable you to monitor playback levels, while clip lights will help you to ensure that your incoming signals aren't too hot. You can adjust tracks' volume and pan positions, and there's even latency compensation that's said to be accurate to within 1ms.
Individual tracks can be transferred to your computer via WiFi, greatly enhancing FourTrack's usefulness as a songwriting tool. This means that you can carry on working on songs started on your handheld in your favourite DAW.
FourTrack is compatible with the iPhone (with the v2.0 software update) and the second-generation version of the iPod touch. It's currently available on the App Store for the introductory price of $9.99/£5.99.