If you're a keyboard junkie and you're not using quicksilver, you're sinning against god :) It's like Spotlight, but better. Also, Spaces is handy if you do lots of stuff at one time (Leopard only).
If you have a notebook that supports multitouch, get multiclutch. It allows you to assign custom keyboard shortcuts to different multitouch gestures on an app by app basis. That way you can add multitouch to nearly any osx app, including those with no multitouch support out of the box.
Only eight hours left on a 50% off 1Password sale. I just picked up a copy because I'm tired of going back and forth with my spreadsheet to get passwords. Just bought it and installed it. Seems to work great so far.