I usually use the proprietary nvidia display driver, but with jessie, I'm using nouveau. It works ok, and with lxrandr, it's easy to configure dual monitors. The only thing missing is an easy, graphical way to change the brightness. So I scripted it. It uses yad.
https://gist.github.com/fsmithred/5a6c0 ... 2c9b3ef0c6
You can adjust brightness and gamma. Set the monitor variables at the head of the script to whatever xrandr gives you as the name of the output(s). See the comments in the script to change it for use with a single monitor, or if you want both monitors to have the same brightness value. (Mine need to be set differently to look the same.)
The gamma setting seems to change on its own if I adjust it. I expected it to stay where I set it. Setting it back to 1.0 works consistently.