del.icio.us is a social bookmarking tool where users can save their internet bookmarks online while sharing them and their classification with others. I love my del.icio.us account. Recently I have been enticed to try out some of the other tools that do this possibly better but another site linked to del.icio.us is keeping me from making the switch, TopTaggers (formerly Collaborative Rank).
Essentially TopTaggers uses the del.icio.us API to track what you are doing on del.icio.us. One of the most interesting (read: addictive) aspects of this is watching your del.icio.us ranking.