Greasemonkey is a Firefox extension which lets you to add bits of DHTML ("user scripts") to any webpage to change it's behavior. In much the same way that user CSS lets you take control of a webpage's style, user scripts let you easily control any aspect of a webpage's design or interaction.[my emphasis]
Since its version 0.2.5 (the installed version on my Firefox is the current 0.2.6) has the added function to do cross-domain xmlhttpRequest. [For the non-technical readers, xmlhttpRequest allows the script to connect to another website and gets back result. Previously, it is blocked by browser because of security concerns.]
A good demonstration of this ability is annotate Google. This user script (as it is called in the GreaseMonkey parlance) adds two additional links after the result title from Google and a xml icon before the title if there is a rss feed. Cool! See the images stolen from the author's site.
There is a great potential in using GreaseMonkey to implement some collaborative learning activities. I shall post some examples here once I have the time to develop them.