If you have an iPad and play guitar, there's a brilliant piece of kit available that allows you to plug the guitar into a variety of amps and effects on your iPad and listen on headphones or, via line-out, into an external amp. It's a fantastic piece of kit for practicing.
The iPad interface is called iRiff port with an iPad connector at one end of the cable and a quarter inch jack at the other. There is a line-out socket at the iPad end and a headphone socket at the jack end.
The iRiffPort works with lots of iPad guitar aps, but the one recommended is Pocketlabworks' PocketAmp. This ap not only provides the amps and effects, it can source your backing tracks directly from your iTunes music and you can play along.
The ap is £2.99, but you do need the iRiffPort, which I got for £79. Well worth the money, I reckon, for a brilliant practice system that would double, in an emergency, for performance through a PA.
Check both out at pocketlabworks
Edit: Don't judge it from the YouTube Demos, they don't give any idea of the real high quality tones you get with this kit.