Clay Shirky surveys the current crop of Web bookmark tools, noting that what's old is new again.
del.icio.us comes out at the head of the class. One thing that would be interesting to know, from a geek perspective, is what API, if any, these tools expose. I'm betting on a burst of action on top of del.icio.us's REST API, but I'm just ignorant of what the other tools have.
Time to do some digging.