Online "SocialSoftware" bookmark managing service. Add URLs to your public collection and tag them with words to categorize them, and see who else has bookmarked that URL; or browse through the site via tag to see what other sites people have categorized. RssFeeds are available for individual users' bookmark collections.
del.icio.us is also useful when you access the internet from multiple machines, and desire to coordinate your bookmarks. When you use different OS's and browsers, keeping bookmarks in sync can be a pain. Delicious provides an easy way out, and provides a very simple and intuitive way to keep track of bookmarks with "tags", which allow you to keep your data in a flat space and invent hierarchies as you see fit. Tags and users can be combined in many ways. For example, you can combine tags as in the following query: http://del.icio.us/KirinDave/programming+design
del.icio.us also offers a (pseudo?) RepresentationalStateTransfer API that any registered user can access. There are plenty of plugins available for most browsers, blog software, etc.