If you're the proud owner of a jailbroken iPhone or iPod Touch, you'll definitely want to check out MooCowMusic (opens in new tab)'s rather astonishing Band (opens in new tab) application, available soon from the ModMyiPhone community source. Offering an easily-expandable roster of sample-based virtual instruments that currently includes drums, bass, piano, guitar, turntable and audience sounds, Band enables you to jam out parts via the iPhone/Touch's touchscreen, with up to five-note polyphony, and - crucially - record and mix the results.
Although by no means the heavyweight sequencing tool we're hoping to eventually see coming to Apple's touchscreen devices, Band looks like a lot of fun, and the ideal sketchpad on which to capture those flashes of musical inspiration that always seem to strike in the pub after six pints...