Looks like all the Amplitude Amp simulators are on sale from $14.99 to $4.99!
I think all of them have a free Lite version which offers one amp to try out and see if you want to buy into the whole thing with multiple amps and stomp pedals.
Note that there are seperate versions for iPad and iPhone so make sure you get the correct one! The links above go to the iPad versions.
I'm tempted by the Fender model ... although I'm pretty sure it's covered one way or another by my many other amp-apps ... oh and that Fender Deluxe amp sitting behind me. Maybe I should just turn that on!