In addition to del.icio.us, there is also a nifty site called nutr.itio.us that automagically manages all of your tag names and gives suggestions for tags that other people have used when visiting a site. Furthermore, links are colored by their relative popularity for that site. Pretty damn cool.
Also there is a very cool Delicious extension for Firefox. You can see it in action below. This makes it so much easier to manage my bookmarks than the crappy hierarchical thing that I had been doing. Yay for good software.
Del.icio.us and nutr.itio.us with some sidebar fun