This second-generation guitar reverb circuit features clipping indicators on the preamp and reverb recovery stages, allowing for the optimal gain settings. The fidelity is much improved over the earlier design, it is suitable for use as a front-end to a guitar amplifier.
http://www.solorb.com/elect/musiccirc/reverb2/index.html
http://www.geofex.com/Article_Folders/wahpedl/wahped.htm
This circuit is a phaser circuit, as used in guitar foot-pedal effects. It used 11 sections of op-amp such as LM324 but others will do.
http://www.4qdtec.com/phasr.html
http://www.solorb.com/elect/reverb/index.html
http://www.neelyguitars.com/Pickups.html
The stock "wah" pedal has been around since at least the early 60's. This thing produces a distinctive tone that is well loved by the expressive guitarist. What a wah does is clear - it is either a bandpass filter or an overcoupled lowpass filter that exhibits a resonant peak just at its lowpass rolloff frequency. The resonant peak can be moved up and down in frequency by the player, and this makes for a striking emulation of the human voice making a "waaaah" tone, or its tonal inverse, "aaaooow".
http://www.geofex.com/Article_Folders/wahpedl/wahped.htm
http://users.chariot.net.au/~gmarts/ampbasic.htm
effect circuit collection
http://www.montagar.com/~patj/gindex.htm