Chrome: If you've ever wanted to see what people are saying about a web page you're visiting beyond the site's own comments, Chrome extension Twitter Reactions will show you the most recent tweets that are talking about the page.
The idea isn't unlike other universal commenting systems, like Google Sidewiki — but instead of just seeing the comments of Google Toolbar users, you can see what the entire Twitterverse is saying about the page that you're on. It takes a bit of filtering with all the retweets or simple linking going on on Twitter, but it's definitely interesting to see what people are saying beyond the site's integrated comments and who's linking to it — especially if it's your own site or blog (you might be surprised how many people have linked to you on Twitter!).
Twitter Reactions is a free download, works wherever Google Chrome does (Note that you need to be on the beta channel for Linux or the dev channel on OS X for extension support). Thanks, Chris!
Twitter Reactions [Google Chrome Extensions]