Social bookmarking through del.icio.us is a concept that is getting much attention these days. The idea of saving bookmarks to a server on the web, so that they can be used from any web connected computer, is not new - Furl, for example, has been around since 2003. What makes social bookmarking different is that you can see what other people have been bookmarking, and let others see what you have been bookmarking. De.licio.us also allows you to annotate and provide subject tags for bookmarks, which is a potentially powerful feature; by subject tagging and sharing bookmarks in this fashion, communities of social bookmarkers will be generating new subject collections of blogs and websites.
I was curious to see how many hundreds of thousands, or at least thousands, of del.icio.us users had bookmarked ircworld. To do that, I registered with the service, bookmarked ircworld (my small but growing collection of del.icio.us bookmarks are at http://del.icio.us/osloirc and clicked on the "1 other person" (?!) who had bookmarked ircworld. I was then able click through to that individual's subject collection of bookmarks.
Another useful feature; when you find del.icio.us bookmark collections that look especially relevant to your interests, you can subscribe to the rss feed that is generated by each del.icio.us page, and thus be alerted to new additions.
In the past I've used bookmarks minimally, relying almost exclusively on search engines to find information. I find however that del.icio.us bookmarklets increase the convenience and utility of adding, organizing and using bookmarks, and the social aspect (along with the inclination every librarian has to build collections) makes it fun! You really need to try it to understand how it works and why it might be useful.
Debbie Weil's deliriously enthusiastic entry about Del.iciou.us at Blogwrite is admirably lucid and an good introduction. For an example of the trail an avid social bookmarker leaves behind, see Weil's collection of bookmarks at http://del.icio.us/wordbiz
Inevitably, there is similar service called de.lirio.us, but I haven't figured out what the difference is.