Chatsum is a Firefox extension that lets you chat and leave messages on any website for other Chatsum users to see and interact with.

The Chatsum sidebar houses a fully-fledged chatroom, specific to the page you’re looking at, and all the other users in the room are also viewing the same web page. When you navigate to a different page the Chatsum room changes automagically. If you open a page in a new tab, Chatsum will keep pace with whatever you’re viewing. There is the option to switch between a page level room and a site level room, and you can also see what rooms/pages are popular with other Chatsum users.

Safari and Explorer versions are in development and the developers, George Grinsted and Lee Parry, are planning some other interesting community features: including “non-chat” surprises, a Dashboard Widget for Mac OS X 10.4, etc.

Check it out, sign up for the beta and help them “squash the bugs.”

More background information: Chatsum Development Blog; George’s and Lee’s blogs.