As mentioned above an Apple iPad can be used as an guitar preamp with touch screen controls....
Here is a small sample of cool amp modelling stuff you can do with an iPad:
IK Multimedia AmpliTube iRig Features:
- Compact 1/4" female input jack to 1/8" female output jack audio interface adapter
- Instrument interface adapter works with AmpliTube for iPhone software
- Works on iPhone/iPod Touch/iPad
- Headphone/Amp/Powered Speaker Out
- Connect electric guitar and bass
Available for both the iPhone and iPad, IK Multimedia brought its amp and effects modeling software from the Mac to Apple’s iOS devices. IK did a great job with the software, giving guitarists great tone. You can also buy the iRig adapter (sold separately) from IK’s website.
Garage Band for the iPad
The weirdest / coolest part of the app, has the opportunity to be your iPad guitar modelling amp. Plug in iRig adapter and the iPad will produce the sound of “classic amps and effects pedals.”