MoonEdit is a collaborative text editor: a number of people dispersed across the internet all work together editing a single text file, all at the same time. It's a cross-platform implementation of an idea pioneered by SubEthaEdit on the Mac.
This is one of those things that makes me think two thoughts: First, "Cool!", then, "What would I use it for?" Has anyone used these editors? SubEthaEdit has screenshots of code. MoonEdit shows IM sessions on crack. What do they really get used for?
Some discussion about the import of this is on CommunityWiki: MoonEdit.