Think you can make a better fast-booting, Chrome-focused OS than Google? Want to craft a custom Linux system that boots from a USB stick? SUSE Studio gives you 15 GB to do exactly that, and you do it all online.
SUSE Studio is what powered the fan-made "Chrome OS" we posted yesterday, which, in that case, was a semi-stripped-down system loaded with the developers' version of Chrome, Google webapp links, and OpenOffice. If speed and cloud computing aren't your bag, you can create a fully functional system with Firefox, 3D graphics, and whatever apps you can find installed. Want your system to start up with an AWN dock and Launchy keystroke launcher running? Not a problem.
Even if you don't know all that much about Linux, it's pretty easy to build a system you can boot from a USB stick or live CD/DVD, run inside a virtual machine program, or actually install it—or, heck, even test it out in your web browser.