Part of the reason I fell behind on writing for New Media Hack, is that I've sort of fallen in love with del.icio.us, a social bookmarking tool that I've come to use more and more recently. As I've said before, del.icio.us isn't all that new as an application, but similar to my thoughts on Bloglines, the difference is in community scale. In addition, del.icio.us, also has a Zen like minimalism that carries over to its REST style API. del.icio.us's simplicity is limiting but freeing.
extisp.icio.us is the first hack I've seen built upon del.icio.us, but there's plenty more potential. For example, internally, del.icio.us could track link copies between bookmarks. The application also supports subscribing to other users. Collaborative filtering anyone?
Also, here's one of my patented half-baked ideas. Provide a calendar based interface on top of a person's del.icio.us bookmarks. Within each day, week, month, provide a global analysis of the URLs that came across the transom. Add some local analysis of the URLs an individual bookmarked to generate a tiny bit of social navigation. For bonus points, implement on top of a jazzy, infoviz toolkit like prefuse to make it sexy.
Anyhoo, there's probably a paper or two in there for some enterprising academic.