quicksilver
I have to admit that I've been using it less of late, though that's started to change. There are a lot of things that I've found are just easier to manage in other ways. For instance, I have two "append to specific files" shell scripts that are aliased in my shell for appending to my todo lists and my "collection notebook" which I find is a lot easier than using quicksilver, and I've gotten into a place where I'm only using a very small number of programs: Adium, MarsEdit, TextMate, and Mail for most things, with NetNewsWire, Camino, and Preview reading/displaying content. I don't really use iTunes on a day to day basis, which was one of those things that quicksilver controls really well.
Anyway.
Cheers!