Roxsyn from Yonac is an iOS synth that you can play using your electric guitar, but it doesn't use pitch tracking, like many past efforts have.
“Unhappy with cold and temperamental guitar-to-MIDI synths that dominate the market, we developed a completely new approach to guitar synthesis,” says Yonac’s chief engineer Jim Yonac. "Our all-original technology synthesizes your guitar directly, without the use of MIDI notes. Rescued from being a poor keyboard substitute, your guitar suddenly becomes an integral part of the synth.”
Roxsyn is available from the Apple App Store for $9.99.