There seems to be a rash of social bookmark/link aggregator services being developed lately. I use del.icio.us, as you can see in the Quick links section on the right, but I also recently discovered Spurl.net. Then there's Furl.net.
All offer slightly different features, differing interfaces and philosophies. Spurl's philosophy is outlined here, for example.
Furl seems to be slanted toward people storing bookmarks for their own
purposes (this is the emphasis in the documentation at any rate), though
they also provide an RSS feed to share your bookmarks, as well as a bit
this returns a table, rather than the more semantically-correct
unordered list or even
Spurl offers very nice integration with del.icio.us. Once you have logged in, you can enter your del.icio.us details in the page for your profile, and then anything you 'spurl' will also be 'del.icio.us-ed' (I really need some new verbs here). You can also import your existing del.icio.us items if you want. It has a nice interface, and if you use IE, Opera or one of the Mozilla family of browsers, you can use a neat sidebar to browse your links and add new ones. If you select any of your own links, you can see a list of links in the collections of people who have also linked to the site you've chosen — a sort of 'if you liked that, you'll probably like these' feature. del.icio.us has something similar but less obvious. The developer of Spurl, Hjalli seems to be keeping up a frantic pace of development, with a lot of recent improvements to functionality and features.
So, my entirely subjective impression is that del.ici.ious is the lean, mean geek machine (a huge base of users, some great links and a stripped-down interface), Furl is for your average Windows user, or people who are looking to store bookmarks for their own purposes, rather than for sharing necessarily (great searching abilities, caching of pages and so forth), and Spurl is somewhere in between (a rapidly-evolving feature set and a prettier face, but fewer users at present).