I recently discovered a site that offers a social bookmarking tool. Del.icio.us enables to create bookmarks easily. After registering on the site, you use a simple bookmarklet to add sites to your list of bookmarks.
But the site is much more than a bookmark list that you can access from any computer. The real genius of the site is the social aspects built in. After you add a bookmark, you may see something like "and 5 others" beside the new site. Click that link and you can see who and when other people bookmarked the same site. You can also click on their username to see what other sites they have been bookmarking - a great way to find people who share your interests (starting to sound like a social network, isn't it?). It can very be a very novel and cool way to browse the web - via other people's bookmarks.
Another great aspect of the design enables you to add "tags" attached to each site that you bookmark. This is a simple space-separated list of words - whatever words you choose. Tags are essentially categories for the links. This is useful for two purposes. One is to categorize your own bookmarks the site lets you few lists of the sites by category. The other that the categories apply globally as well - so you can see the sites that others are bookmarking in the same categories as you - and others may discover your bookmarks while browsing the category lists.
Bookmarks are also available via RSS and APIs, making the site even more useful.
The idea of socially bookmarking, combined with the simple easy-to-use design of the site, is just brilliant. What a great idea.