del.icio.us is a social bookmarks manager. It allows you to easily add sites you like to your personal collection of links, to categorize those sites with keywords, and to share your collection not only between your own browsers and machines, but also with others.
Mine's here: http://del.icio.us/nascent
Very handy... also a good read when bored :-)
I find one of the best features del.icio.us has to offer is the RSS feed. In firefox, simple click in the [RSS] label that comes up in the lower right hand corner, and you have a "live bookmark" ready to go!