del.icio.us

From the site:

"del.icio.us is a social bookmarking website -- the primary use of del.icio.us is to store your bookmarks online, which allows you to access the same bookmarks from any computer and add bookmarks from anywhere, too. On del.icio.us, you can use tags to organize and remember your bookmarks, which is a much more flexible system than folders.

You can also use del.icio.us to see the interesting links that your friends and other people bookmark, and share links with them in return. You can even browse and search del.icio.us to discover the cool and useful bookmarks that everyone else has saved -- which is made easy with tags."