A Chromebook is a notebook computer that runs the Chrome OS and its only function is to access web-applications via the Chrome web-browser. Chromebooks are manufactured by Acer and Samsung. They are sold at Newegg, Tiger Direct and Amazon. They are priced between $349 (Wifi only) to $499 (with WiFi and 3G connectivity). Amazon and NewEgg always seems to be sold-out so go to Tiger Direct.
Build Your Own Chromebook
I do not have a spare $500 or even $350 so I made a Chromebook from my HP laptop. You can do this with any computer you have laying around. A 1GHz or better processor and 512MB of RAM should get you started. You can only install Chrome using a USB stick but if your computers BIOS does not support booting from a USB stick you can download the Plop Boot Manager Live CD that will install the USB drivers so you can boot from the stick.
If you do not have a spare computer or you just want to experiment with Chrome OS you do not have to install it but you can run it from the USB stick.
The OS that you will install is not really the Google Chrome OS that is installed on the Acer or Samsung notebooks. It is the open-source brother of Chrome call Chromium. It mostly looks and acts like Chrome OS and the two projects share their work with each other so they really are the same.
Make sure you install Chromium OS Lime and not Vanilla. Lime has extra hardware support and is not as likely to have compatibility issues.