One of the oldest -- yet somehow least-lauded projects in existence -- is Mycroft. Perhaps its under-hypedness is due to its total simplicity -- Mycroft is nothing more than a massive directory of search plug-ins that you can add to Firefox, Chrome and Internet Explorer. Check the top 100, and you'll get some idea of Mycroft's scale.
For Firefox and Internet Explorer, this means you can search just about every site in existence from the Search bar in the top right corner of your browser. For Chrome users, this means you get even more functionality from the uber Omnibar.
Incidentally, Chrome users, did you know that you could use specific search engines from the Omnibar? When you grab a search plug-in, make sure you provide a keyword. Make it something short -- like 'pb' for Pirate Bay or 'imdb' for IMDb -- and then, when you want to search The Pirate bay, just type pb, followed by your search term.