Ooh Wah 2 is a ton of fun. The tweak from the original that reset the clock made it for me
Frantone Glacier is very cool as well. I didn't have much use for it back in the day, but my drummers foray into electric kits, dub step and the like has me wanting for some mad synthy pedals. this would be top of the list.
HOG. Only just picked one up and I've already put it to the side until i have a good week off work to nut this thing (or less nights in sweltering heat where i can sit in the studio and jam to the wee hours).
Zoom G3 ... because it's got approximations of virtually every effect you could possibly think of. May not pass cork-sniffer muster, but it's a good way to get a feel for what all kinds of effects sound like.
For about $100 used, you've got access to so many different effects and possibilities. Need some more dirt? Done. Need some eq and compression and noise reduction? Done. And the list goes on and on.