A pretty cool way to keep an eye on those Twitter users you’re vaguely interested in, but not to the point of actually following them. I had an “open in tabs” bookmark folder in Safari, but it made the browser lag to open all those tabs; this lets you group users very easily and display them all in a single page. I’m going to unfollow a bunch of people today.
No registration or anything, you can create as many bookmarked groups as you like by just adding user names to the URL. As long as the site is open.