Jolicloud is the closest operating system to Google's Chrome OS that you can currently download. Besides a beautifully designed user interface, it's extremely quick (with a clean install, you can boot up your netbook in seconds), and extremely easy to use. Powering Jolicloud is a combination of Chrominium (the open-source browser Google used to build Chrome) and HTML 5, so like the Chrome OS, it's basically an always-online OS.
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