JotSpot’s 20% day
As Mike points out in comments, Abe Fettig's collaborative editing is just brilliant. Here's the demo. Looks like it uses some Ajax magic, including something similar to 37Signal's Yellow Fade Technique. Very slick. Having used SubEthaEdit a few times, I know how useful this really is, in the right circumstance. It takes the wiki a whole step further towards true collaboration.
Also, see a few more hackathon photos.