That’s My Mouse allows you to see share a web browsing experience with other people. You see their tag moving around as they move their mouse and any changes they make to the page. You can also add pins, write comments, etc.
They seem to be keen for people to try mash-ups with it and use Google Maps as an example of usage, showing someone the way to your house. But I think it could be pretty useful for web designers working remotely to easy collaborate and talk about changes.