The iRig Pro is what I use, but if you don't need/want MIDI the iRig HD would be fine.

Other options are the Line 6 Sonic Port, Apogee Jam, Griffin GuitarConnect Pro, AmpKit LiNK HD, Alesis ioDock, as well as the above-mentioned Griffin StudioConnect.

But whatever you buy make sure it has the dock connector (CCK on older models, or Lightning on newer), as opposed to something that uses the headphone jack as an input.