There's also PangoBright, but it's not automatic, it just sits in the system tray and you select a brightness level manually.
I think it's going to be hard to achieve varying degrees of brightness through software without completely messing up your TV's picture quality - when you're making adjustments you normally start by getting your black levels right and work from there. You really want something that adjusts the backlight directly, which means having the functionality built into the TV itself as mentioned above.