Vocoders and Kaoss pads would be obvious choices in the industry. I'm just taking a look at pedals that is usually meant for guitars, stacking and switching as you would on a pedal board, but putting it into vocal context, since they are less obsure, more readily available and possibly are cheaper alternatives.
Some pedals I feel like trying (only a few I have tried):
overdrives - probably not very out of this world as compared to a vocoder, but sometimes just a slight coloration of dirt or drive from a Tubescreamer for example, can be interesting for recordings, I would think.
Would mean squat in live situations, though.
Distortion - Metal Zone sounds great on drum loops for sure. Gives a very industrial, NIN-like feel, heh. I have yet to find a good usable setting for vocals. Still trying.
Whammy - In fact, everything on Digitech Whammy seems like a great idea to experiment with, heh.
Auto wah - Less control than a normal wah, so less predictable. Which can be a good thing.
Loop Pedal - something I would definitely love to try and see what it is capable of, especially in live situations.
Maybe a bit of problem since the pedals' inputs/ouputs are 1/4" jacks as oppose to the mic's XLR. Nothing a proper adapter can't solve, though.
Of course, there's always this at Swee Lee, but I kinda think it's an overkill:
DigiTech? Vx400