I'm not sure whether or not this is a really cool thing, but I think it might be. It's del.icio.us, and it's a "social bookmarks manager". It's nice to have a web-based bookmark manager, as I found when I wanted to take my work bookmarks with me when I left my last job. But it's also kinda neat to make them public, (mine can be found on this page), and see what other people are linking to. And see how many other users have linked to a page that I'm interested in. And go to one of those user's pages, and see what else they might have that might be interesting but not popular.
Another nice thing is that it lets you categorize your links, assigning any number of keywords to each link, and then pull up just links with one or more specific keywords.
So, it's simple and useful, and public. I'll declare it really cool as soon as I figure out whether I like the public aspect, but I think I do. (Go ahead, Rob, talk me out of it...)