Wanna give Google Chrome OS a try here is a nice option to install the OS on your USB pen-drive without the hassle of compiling your own distro from source-code, thanks to ChromiumOS Cherry you can now download a pre-compiled build of Google Chrome OS ready to install on your USB pendrive and boot directly into Chrome OS on any machine capable of booting from USB.

To get started all you need to do is to follow the easy instructions for - Windows/Mac/Linux, also don't forget to read the FAQ to make things even simpler.