ShiftSpace is an open source platform that uses Greasemonkey to place an API layer over the web. When you have the plugin installed, ShiftSpace connects back to a central server to locate “shifts” that other users have left behind for the URL you are viewing.
By pressing shift+space on your keyboard, you get a list of these “shifts”, which are essentially a layer of user-contributed annotations that are layered on top of the web page you are viewing. The current tools allow you to leave post-it notes, swap images, highlight text and even reformat the html of the underlying page. To add a shift to any web page, you just hold the shift key down and a little menu appears.
Right now, the shifts you see appear to be global. In the near future, it’s supposed to have the ability to filter shifts based on friend groups, friends of friends, or just you own shifts. This could make it a convenient way to take notes on pages that you view, alone or collaboratively.
The bigger deal is that the underlying API is open source, and the tools provided are just examples of what you might be able to build. The server-side is written in PHP using the SQLLite library, so it should run on just about any web server environment. If you want to hack a meta-web application of your own, you can just download the source and start coding. Just make sure to tell us about it when you’ve got something to show.