since i had the setup from my stupid discovery topic ( http://www.diystompboxes.com/smfforum/index.php?topic=57774.0 ) on top of my amp i tried something different too.
I hooked up a piezo as a speaker to a small amp and soldered the end of a spring to that piezo. then i hooked up another piezo to the input of my marshall and isolated that with a piece of tape. i attached the other end of the spring to the taped piezo with a paperclip.
so the setup is: guitar - small amp - piezo - solder - spring - paperclip - taped piezo - normal amp
this gets you a kind of lofi reverb, i think if you fool around with it a bit you can get really cool sounds. it also does the thunder thing when you hit the spring :p
I believe they do use actually piezo transducers for some of those larger plate reverbs
Piezo reverbs were used in the JMI Vox Conquerer, Defiant, and Supreme amplifier heads.