I got my hands on an early build of the Web operating system Jooce this week. I find this class of product interesting, but I've yet to see one that's really compelling. See also: Startforce (video), YouOS (review), DesktopTwo (review), Glide Effortless (review), and Goowy and YourMinis (review) .
The concept is elemental Webware: Web OSes, at least in theory, put all the heavy lifting an operating system usually does on a server somewhere on the Net, while the interface gets funneled through a browser. With a Web OS, you don't need to worry about any additional software or storage on your local computer. This means any browser-equipped machine can become your personal machine with a simple log-in. And since all of your data is stored centrally, it's also generally easier to share and collaborate with others.
The downside is speed and the availability of apps. Jooce, in its current, pre-pre-release stage, is a Web OS that appears to have done a good job solving the speed issue. But I didn't see any serious applications to work with it.
The Jooce UI is built in Flash and is nice-looking and easy to use. It's also got a clever sharing feature: Each user gets two workspaces by default: private and shared. You can easily move files between them to make them available to others, and a quick mouse-click rotates your workspace between the two spaces, much like user switching on Mac OS. (In fact, the default desktop background is a pretty blatant ripoff of the blue swoosh on the Mac desktop.)
At the moment, there are no Jooce apps to play with aside from a multi-service IM client, a file uploader, and some media players, but Jooce CEO Stefan Surzucki told me that their platform enables apps to easily talk amongst themselves, so you won't have the collection-of-separate-widgets that you get on single-page aggregators like NetVibes. But somebody's going to have to come up with good ideas to implement, and then build those apps. I can't tell yet if that will be easy, or hard, or if anyone is yet lined up to do so, especially considering the explosion of app and widget platforms that developers now have to worry about.