Citeulike has some really great qualities -- like the fact that you can see what people are reading. But, it's sort of annoying to get citations into CiteUlike sometimes. The cloud of tags on the right of the main page shows you how popular given tags are. The bigger the tag, the more items it contains.

"This is Connotea, a free online reference management service for scientists created by Nature Publishing Group...Connotea helps you store your reference list online, which means that it's readily accessible, it's linked directly into the literature and it's easily shared with your colleagues. Opening your references to other researchers enables you to discover new leads by connecting to the collections of those with similar interests to you.

Web application LibraryThing lets you catalog all the books you own and use tags to organize your collection.
Add book titles by entering a title and viewing search results from the Library of Congress or Amazon. LibraryThing adds the book’s card to your catalog with ISBN, publisher, year and an image of the book cover. You have space to add a book summary, tags, your comments and a review. See what other users also have each book in their library and what they’ve tagged it. LibraryThing is an impressive cataloging app that feels like del.icio.us for books.

Flickr allows users to store and organize their own images, and to view, tag, and comment on other immages.

It takes a little getting used to, but Delicious is an incredible tool for keeping track of websites and finding other ones. I never really bookmarked things before delicious, but after a little practice, I can't imagine life without it. I have feeds for my favorite tags (like one for GIS) so I can see what's happening in the web world. Because of the interests of the people using it, it still tends to be pretty techy in the links in there, but more and more people are using it all the time, with their own interests. It's worth trying to get to know it, I think.