Prayaya V3 tried. 
Basically,it is virtual operaing system that allows you to launch most apps and games off a USB drive BUT not officially claimed any app and game. 
To use it, you first install it on a removable device like USB drive, iPOd, or mobile HDD, or even your hard disk(I did not try it, but it seems ok.). And then you can install ware from Prayaya V3 platform. 
In my opinion, it virtualizes a new Windows OS, which makes it possible for installing and running apps, managing files, desktops etc. 
A few apps and games that I have tried on it: Firefox, Opera, Safari, MS office, Nero, Winrar, Virtual DJ, Skype, Plants vs Zombie, Icy Tower, Super Mario, Couter Srike, KM Player, Yahoo Messenger, MSN, Picasa. These are my daily used apps. And they work perfectly. 
However, there are some that seems not working on it like Google Chrome, PhotoShop, Coreldraw.
Also I found that Prayaya V3 seems to write registry into USB but not a host computer. I think that is why it can launch so many apps and games. a good advantage is that not any trace will be left on a computer if you run it off a USB drive. 
Generally, I think it is a cool ware that is worthy of a try. I like its cool interface, cool feeling switching between two systems, two desktops. However, if you just want to run common apps off a USB drive, I think PortableApps.com is enough for you. If you want to go furthur, Prayaya V3 is a good try. 
Here is the official homepage of Prayaya V3: 
http://www.prayaya.com