If you've got numerous apps for guitar effects but want a more organic feel, then check out the StompBox iPad.
The Stombox iPad is an actual multi-functional foot controller designed by Griffin Technology and Frontier Design Group. You can program your favorite effects on to the board and activate it via foot control. There's even a jack input for additional plug-ins such as a wah pedal. The Stompbox iPad is expected to be released some time this year.