You know the one--not the toolbar, but the button that you click on your Links bar that opens up a small dialog box where you can enter search terms, or if you highlight some text on a page and click the Google button it will Google that text (the button you can install from here:
http://www.google.com/options/winexplorer.html">
http://www.google.com/options/winexplorer.html.
Anyway, you can make that button search anywhere you like. I have the Google button, and two others that act the same way with Wikipedia and Dictionary.com.
ON EDIT:
Ok, I can't seem to post the code, but if anyone is interested I can email/IM/whatever it. It's very simple, really!
Once finished, you can highlight a word on a given webpage, click the Dictionary button, and it will take you straight to the entry in Dictionary.com (and will do the same for Wikipedia if you click that button). A pretty handy browser customization!