A lot of people use GemFX, SweetFX or ReShade, all of which as far as I've seen have been green-lighted as they do not break the TOS. They make no changes to any of the games files and offer no cheats.

@Jacosta, I find that it really doesn't tax my system at all. I have a decent rig and can play on with everything on the highest setting and seem to maintain 60+ FPS at all times. I don't know how your system is set up, but you could always give it a go and see how well it works for you. It's easy to remove if you find you don't like it.