A guitar synthesizer? Like something that you plug a guitar into and it can make it sound like a sax or a piano or something different?
My Stuff!

Yamaha Pacifica 112
Vintage V400-12 Solid Top 12 STRING!!!!!
Yamaha F-310
Yamaha FD02 Solid Top
Orange crush 10
Watson XL15
Zoom G2
Encore PK40

Saxophone: Jupiter somethin or other
Check out the Roland if you want a very powerful synth. If you just want something basic with attack delay and octaves then check out the POG or the Micro Synthesizer from Electro Harmonix.
My Blog
New bands you wish you knew about!

Check This Band:As Blood Runs Black
Guitarist of the month: Quorthon

Got a good band that you want to share with the world? PM me and I'll write them a review.
I know that there are some synths that use guitars as midi controlers but if your just trying to get trippy, Digitech makes a synth wah that has several different modes like an envelope filter, filtiering in both directions, and some other really cool ones. go to their website and there are some sound clips. I use loop pedals and a ton of stompboxes to layer up trippy sound textures. I use delay set to overlap/machine gun, phasers, and flangers just as much as the synth to create intersting synthy sounds. Step functions on phasers, gates on tremelo or flangers in combination with a delay of the same time interval,wah pedals, and whammy bars can all do really interesting stuff.
You need a MIDI pickup, a MIDI controller, and a synthesizer. The simplest solution is a Roland GR-20, as it comes with all three, but if you buy them all separately, your options are almost limitless. Technically, you could buy a pickup and controller and hook your guitar up through a Korg OASYS (if you're looking to spend $10,000)
Someones knowledge of guitar companies spelling determines what amps you can own. Really smart people can own things like Framus because they sound like they might be spelled with a "y" but they aren't.
Maybe some sort of Moog MoogerFooger? Look into those.
Do YOU know who Les Paul is?

-Epiphone Dot Studio
-Fender Stage 112 SE
-BBE Soul Vibe
-Boss OD-1 Overdrive
-Ibanez DE-7 Delay