Quicksilver
If you own a Mac and haven’t tried Quicksilver, you may be doing yourself a profound disservice. Quicksilver is a smart application switcher / desktop workflow tool. Here’s a better explanation.
I’m pretty much using it for doing simple stuff like opening apps and finding files, but it seems that once you achieve Quicksliver guru status, you could rip through reasonably complex multi-step workflows like kids pull off long Tekken combos, in that the heavy lifting can be relegated to muscle memory.
After using it for a few weeks now, I’d have a real tough time getting rid of it.
I just installed the Cube interface, and although it’s arguably just eye candy, that UI seems more Quicksilver-ish (or true to itself) than the other interface options. Here’s a movie of the cube in action, recorded by Tim Gaden at Hawk Wings.