This is Radiance, a free, fully standards-compliant CSS template designed by Free CSS Templates. This free template is released under a Creative Commons Attributions 2.5 license, so you're pretty much free to do whatever you want with it (even use it commercially) provided you keep the links in the footer intact. Aside from that, have fun with it :)
This template is also available as a WordPress theme at Free WordPress Themes.
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