Headup is the latest company to attempt in-browser semantic search. The Firefox-only add-on that, surprisingly enough, uses Microsoft's Silverlight to do its heavy lifting, will scan whatever page you're on and search a cross section of the Web for related news stories, music, videos, and more.
To personalize the results it finds, you can plug in your contacts and accounts from various social services including Twitter and FriendFeed. This will pepper the results with related tidbits from them when applicable. For example, a quick Headup search for "cheeseburger" showed me that one of my FriendFeed buddies had posted a shot of his late-night meal from the weekend. That's a little creepy.
The search itself takes only a few seconds, since results are streamed as they come in. It's very slick looking, but I found the results a little hard to parse through. Its creators have gone with a strange design that loads up a very small preview of any photos or text, which almost always requires clicking to go to the source site.
The design's not a complete showstopper, but I found myself not wanting to go much further than story headlines and photo previews because doing so would have required loading up an extra page. If the interface could be a little simpler to parse, and resemble something like Google Reader or iTunes with a more approachable source list or results, I think I'd dig it.
Another company doing something similar to this is Vysr, whose RoamAbout add-on puts contextual search right on the side of your browser. It's not as social as Headup, but is slightly less intrusive in its presentation.