One of single (test) most productive tools for a mac is Quicksilver which is a free plugin that allows you to quickly hit a few keys and conjure up all kinds of wickedness from your system. And now it looks like Ubiquity brings this genius to the Firefox browser. Check out the video below for a demo and here is the link to go get it: http://labs.mozilla.com/2008/08/introducing-ubiquity/
Ubiquity for Firefox from Aza Raskin on Vimeo.