Synth pedals have become quite popular recently. They obviously let you turn your guitar into a synth but they're usually a lot more than a synth keyboard simulator. They let you almost turn your guitar into a new instrument having dozens of knobs and sliders to experiment with.
Synth Pedals
EarthQuaker Devices EarthQuaker Devices Data Corrupter Guitar Synthesizer
£255.00
Electro Harmonix Electro Harmonix Guitar Mono Synth
£118.00
EarthQuaker Devices EarthQuaker Devices Bit Commander v2
£189.00
Source Audio Source Audio Spectrum Intelligent Filter
£225.00
Keeley Keeley Synth-1
£179.00
Electro Harmonix Electro Harmonix Bass Microsynth
£261.00
Meris Meris Enzo Synth
£319.00
Electro Harmonix Electro Harmonix Mel9
£205.00
£475.00
Electro Harmonix Electro Harmonix Voice Box
£219.00
£135.00