LaunchBar - I appreciate Quicksilver, but I've defected to this. If you're not reliant on a specific plugin or advanced feature of Quicksilver, then, IMO, this is a tighter and superior program.
Perian +
Flip4Mac - There's no need to install an additionnal video player. Add these components to Quicktime and it will handle the various formats system-wide.
Unarchiver - For decompressing files. It's like Stuffit Expander, but more streamlined and more reliable.
SteerMouse - I wouldn't own a Mac if an application like this wasn't available. Makes the mouse pointer behaviour usable.