I wasn't sure what it was when I first saw it or even if I would use it much. But after I used tags with Flickr it clicked and I went back to del.icio.us and started bookmarking things there and it's helped me to find stuff later. The neat thing is to go through the tags and to see how many other people have bookmarked what you have. But even if I just use it to keep track of interesting things for myself, it's still great. I add to my del.icio.us bookmarks almost every day now. I think of it as microblogging - blogging with minimal writing with the focus on categorization.