del.icio.us
Only recently did I discover "del.icio.us":http://del.icio.us/ and now I know what I've been missing. They say about themselves
del.icio.us is a social bookmarks manager. It allows you to easily add sites you like to your personal collection of links, to categorize those sites with keywords, and to share your collection not only between your own browsers and machines, but also with others.
I'm not so hot on the _sharing with others_ bit, but I use _del.icio.us_ for sharing with _myself_ (i.e. between all the computers I use in distinct locations). Oh, and they give you an RSS feed of your own bookmarks; invaluable. The best is the little bookmarklet which you just click on to save a bookmark and give it some meaningful tags. _del.icio.us_ is currently free of charge.