YooNo is a useful browsing utility that combines several social features. On any Web site, it shows you: other sites that were liked by people who bookmarked that site; blog posts and articles related to the page; and info on the site's primary users (called "Yoosers").
It could be an overwhelming amount of information, but it's actually pretty helpful. The Links (related sites) function is both the oldest YooNo feature and its most useful. Based on YooNo's shared-bookmarks function, it helps you find sites related to the one you're viewing. It's a collaborative filter, in other words, but you don't have to know that to use the site. And you don't have to collaborate, either--YooNo returns results even if you keep your own bookmarks private.
Coming soon (within a few days) is a new feature that will show stories and posts related to the page you're on. This is based on searching and link analysis, not analysis of bookmarks, and is reminiscent of Sphere. It could become a good site for picking up the newest news on pages that interest you, although in my tests (using a beta) it didn't reliably return good info.
Finally, the "Yoosers" section will soon show you the people who bookmarked the site you're on, and let you can drill into a user's public bookmarks to explore a topic in more depth. You'll be able to pull up a very fun, floating-bubble view of the network of users around the one you're looking at, based on shared bookmarks. Frankly I find it incomprehensible, but it sure is fancy. MyBlogLog does a better job of creating ad-hoc networks around Web surfing behavior.
The service also lets you synchronize your bookmarks across multiple computers, a handy feature.
YooNo works as a plug-in sidebar for Firefox (IE is coming) and is a worthwhile download based on the related-site feature alone. The related-articles feature could also be very useful. I don't know what to make of the Yoosers function.