The search dropdown feature of firefox has never been one of my favorite features. It just takes too long to switch between them, so I've mostly left it at google, and hoped it would be smart enough to understand what I mean. However, with linkhack, it just proxies through to google most of the time, but understands keywords such as 'imdb' to do a movie search, or 'img' to do a image search.
I'm quite pleased that they are using iusethis to 'get' an application as well (try 'get firefox', for instance). I can't help but think that this maybe should be implemented in the browser itself rather than as a web application, but it's definitively an interesting concept. one thing that worries me is if I'll be wanting to google those magic keywords some times. Of course, you can always prefix queries with "google".