There definitely are.
The two guitar amp modeling programs that are really big are Native Instruments' Guitar Rig and the other is AmpliTube. I don't know about AmpliTube, but with Guitar Rig you can get some really space-aged sounds out of it. They basically work as amp modeling software, but you can filter them to sound like a synthesizer (with chorus, flange, phasers, etc.)
Thing is, with just about any guitar synthesis set-up, there's the issue of latency. They don't process guitar sounds in real-time the way a keyboard does. This is definitely true of MIDI pickups. The Stealth Plug (a guitar cord that hooks up the guitar to a USB port) is supposed to be able to do this, but I don't know for sure.
Honestly, your best, sure-fire bet for getting synth sounds in real-time is going to be stand-alone effects units. I own a really cheap Ibanez CF-7 flanger pedal that can make TONS of weird synth-esque noises and it's way more versatile than any amp-modeling software. I bought it for $20 at a pawn shop. On the other end, I bought a Moog MoogerFooger for five-hundred dollars, and it works wonderfully for turning the guitar into a synthesizer, complete with the LFO's and Resonance filters you'd find on a classic synthesizer. Moog makes a complete line of guitar pedals that work wonders, but they're expensive as hell.