Holy crap! Google launched Blogger Web Comments, something I've been imagining for years (and built in a non-scalable prototype way in 2000). It's a Firefox plug-in that uses blog search to show you the latest stuff people are saying about the page you're currently viewing.
It also has a posting feature, which works if your blog's using Blogger, but that's not really the killerest part anyway, if you have a BlogThis!-like thing. (In fact, there's some problems with the posting.)
The killerest part is the reading. How cool to be checking out a new web service, a company, or a product and instantly see if there are any web posts linking to it? (See screenshot.)
It's really well done, too. Check it out. (Nice work, guys!)