...and now Ableton Link! Massive value if you havent purchased this one-of-a-kind app yet. I use Jam Maestro whenever I want to do some serious song writing and dont have my guitar.
Its not easy getting an authentic sounding electric guitar sound on iOS without the guitar, but Jam Maestro comes closer than any of the other apps I have tried, and Ive tried almost all of them. Add to that its feature packed sequencing and you have a powerhouse tool.
ZenLizard about Jam Maestro: create guitar tab