Visor

Boy, those Blacktree boys write some good software (you know, like Quicksilver). The latest gem I've found is Visor, which mimics the console found in lots of video games, but for the entire OS, using Terminal. If you spend a lot of time in Terminal, Visor allows you to drop a terminal window down from the top of the screen with a hotkey, then use the same hotkey to slide it away again. It takes a couple of steps to install it, but man is it cool.