Chrome has a great feature that lets you select text and right-click it to search that term on Google. Drag & Drop Search is an extension that does something similar: You select text, then drag it somewhere on the page to search up to 16 different websites.
Once you select your search terms and drag them, a bunch of blue boxes will appear around the page, each corresponding to a different custom search engine. That way, you can search for those terms on Google, on YouTube, on Wikipedia, on Flickr, or any other site you could possibly think of, with just a click and a drag. You can add your own custom search engines and assign them to anywhere on the page, too.
What's really great is that the extension stays out of your way until you need it. It doesn't clutter up your context menu, toolbar, or anything else -- it only shows up when you want to perform a search. Hit the link to check it out.
Drag & Drop Search is a free download.