http://mycroft.mozdev.org/ has a bunch of useful search engines that can be added to the Firefox search bar (and other Mozilla browsers, i.e. SeaMonkey) pull-down list. I just added Digikey and Atmel to mine.
Unfortunately there isn’t a huge complete list of all the engines that have been submitted for me to scroll through for hours, so if you find more good ones please post them!
-Ada