Amplitube 2 by IK Multimedia is an awesome little app that allows you to convert your iPhone into a full on guitar rig. Their iPad version was recently featured in one of the new Apple iPad ads. The app has 3 versions currently - Amplitube Free, Amplitube LE ($2.99), and the full Amplitube version ($19.99). We are using the full version for this review to give you a complete picture of what's available.
To use this app, you'll need to purchase an iRig, which are currently priced at $39.99. You'll simply plug your guitar or bass into the iRig which then plugs into your iPhone or iPad. The 3rd outlet allows you to plug into the audio output of your choice. We just used a standard surround system (receiver) for this review. You're also free to use an amp or headphones, which can be great if you'd like to practice quietly. And hey, it's a lot easier to carry around the iRig than an entire guitar rig.