iTunes link: GarageBand - Â£2.99
GarageBand is Apple's own simple music sequencing package. The iPad version offers a handful of virtual intruments you can play right no the device's touchscreen, including drums, a piano and guitars, but it's also useful for "real" musicians.
Buy the right additional cable and you can plug a guitar or microphone into the iPad, turning it into a mini recording studio. It's not going to go rivalling Pro Tools any time soon, but it's perfect for use as a musical notepad.
Plug-in a guitar and the iPad also plays the role of the amplifier and effects box, with nine amps and 10 effects boxes included in the straight Â£2.99 package. Even if you have no intention of taking GarageBand's skills seriously, its virtual instruments are still fun for a quick noodle.