del.icio.us, owned by Yahoo, is intended to be used as an organizing tool for storing bookmarks to your favorite websites, music, and books and also to share them with family and friends.
In my case it's more like a dis-organizing tool.
I look at the front page links and end up reading about bearskin rug designs, or how accordians are made, or where zirconium comes from...the kind of stuff I need to know ;-)
In other words it's also an interesting place to browse around and discover interesting things.