Agile Partners have made a number of great apps for the iOS platform, including GuitarToolkit, TabToolkit, Guitar World Lick of the Day, and AmpKit. AmpKit is an amp and effects modeler, so you can plug your guitar directly into your iPhone or iPad and rock out. The following is a video of Alex Skolnick demoing the app:
It really is amazing what this app can do. I recommend checking it out if you have an iOS device.