Can't take credit for this one, saw it over at NoInstall.com but it's certainly not a concept that I've heard before: Having a desktop environment in your browser. It is kind of neat but can be a little frustrating since it is/can be sloooow (and I'm on a fast cable connection!)
I've tried (very briefly) both the suggested sites mentioned in the post and they are pretty impressive. At the moment I'm leaning towards eyeOS simply because it's a little leaner (which translates to faster), however YouOS have done a good job of trying to incorporate that traditional desktop feel. Most of the apps themselves are pretty basic (I suppose you can't get too complicated when data has to sent back and forth across the Internet) but for some simple day-to-day tasks, it might be sufficient.
Naturally the biggest advantage is major portability, not just in terms of having access to 'your stuff' on the move but also being able to access it across multiple platforms (any OS with a standards compliant browser... guess that's Internet Explorer out the Windows then). The nice thing being that you don't have to carry anything with you at all. However, I would have some concern about security, speed and reliability, and the fact that if the server goes down then you are in some trouble (of course, you could argue that the same thing is true if your flash drive/HD gets corrupted. However, in this example at least you have some control over data recovery steps).
For me, I''m still not convinced that I need a whole mobile desktop. For the most part certain functionalities are available in standard web portal accounts like MSN/Hotmail and Yahoo. For example they both offer email (of course), calendars, file storage, bookmarks and various other featurettes. However, I can appreciate the convenience of having access to all of that from within a familiar desktop like environment.