Another, although much more "pumped up" extension that will do the job, is Fastest Search. If you already have installed it, but didn't know about this feature (as I did, before coming here), you can use the shortcut "gsd" (Google SubDomain) in order to search with "site:".
I found the extension when trying to resolve an issue I had with InstantFox and discovered it is a lot better for me (except for the visual part:). It did not actually solve my issue, but that's another story... I just give the InstantFox example, because most people know this extension and can get an idea what Fastest Search is about.