Communications & Ear Protection, Inc.
Can be fitted to helmets or headsets, very reasonably priced, and has stopped a number of us at Waltham from going deaf. Works in tandem with the headset or helmet speakers, so you can choose not to wear them if you're flying something less noisy once in a while.
In the Pitts ANR was simply overwhelmed by the noise levels - a mate who had factory fitted ANR in his Gentex gave up on it and went CEP, I also used in my Gentex, and another mate went for the rather more expensive option of having custom moulded earplugs made, operating in the same fashion as the CEP system, in his Alpha.