I started bookmarking using http://del.icio.us recently. Awesome site!
It's displays several trades of an excellent web-application; It's simple, user driven and bloat-free.
What really made me realize how powerful http://del.icio.us is, was an excellent podcast from http://www.itconversions.com called "Ontology is overrated" by Clay Shirky.
An ontology is a categorization of things that exists. The categorization tends to take on a dominating perspective influenced by the authors of the ontology.
For instance, the books in a library is sorted by author rather than by color. So if you're looking to find a specific red book in a library (and you don't know the author), you're out of luck. That's not a supported way of thinking :-(
Of course, computers and networks and especially the web is excellent for cross-referencing things. So there's no reason at all why you shouldn't be able to do this. It's just that it didn't occur to the people designing the system that you would wan't to do strange stuff like that.
http://del.icio.us is sort of a dynamic ontology realized by having users share bookmarks and apply tags (keywords) to those bookmarks. And yes, this is categorizing, but the users define their own categories. Categories form where different peoples bookmarks intersect. These are also intersections of peoples interests and by copying other peoples bookmarks you're exercising and forming social relations.
Del.icio.us builds the ontology buttom-up instead of top-down.
One of the interesting things Clay Shirky talks about is how ontologies tend to degenerate over time. This is bound to happen because the world of existing things on which the ontology reflects, changes. Categories are collapsed and information made up by their difference is inherently lost. Del.icio.us doesn't collapse categories, instead new categories becomes popular.
Instead of following the traditional model of trying to force users to use and adopt an ontology (like yahoo groups and the library does), del.icio.us is an adaptive and more useful ontology.
Found an interesting article on social bookmarking tools.