Guitar effects are awesome. As everyone knows, though, there is an absurd number of guitar effects out there. With the great number of different guitarists here, I want to hear from a bunch of different people what their favorite effects are. I know that this is vague, and that there are a great number of different categories and types of effects out there, but I just want to hear everyone's opinion, from pedal effects to rack modules to multi-effects boards. It might help to do something along the lines of "envelope filter - Doctor Q" (that's one of my favorites, just because it sounds funky and you can buy one for $40) to clarify things.
But yeah, there are so many people here, I'd love to get a bunch of opinions on effects under one topic.