It's in the Trisquel repos; Trisquel only supports free software, so I assumed it's free. But it's probably best to look try and check indepednently. I found the official site and the wikipedia page:

http://www.x.org/releases/X11R7.6/doc/man/man1/xbacklight.1.xhtml
https://en.wikipedia.org/wiki/Xorg
http://www.gnu.org/licenses/license-list.html#X11License


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