Any kind of the retribution being talked about ITT (telling the other guy's wife, kicking the other guy's ass, kicking her out of the apartment, posting her nude pics. whatever) is going to be totally ineffective and probably backfire. It's just going to piss her off. Hell, if you want to piss her off, just set her clothes on fire or something. She'll still get pissed, you'll still come across like an *******, and in her own mind she'll easily convince herself 'see, he's just an *******, I needed away from him anyway.' And what she thinks inside her own mind is all that matters.
The cruelest thing you can do to her is break up with her and never mention the cheating at all, act as if you don't know about it, act surprised if anyone ever brings it up to you. Then go out without missing a beat and smile your way through life and pick up another girl or two. She WILL get seriously ****ed up by this. It's just the way women are wired. If your whole attitude is 'I'm dumping you because I can do better' she'll torture herself over it.
If you want revenge, that's the only way to do it. Anything else is gonna backfire like I said, because it'll just make you an ******* and her justified in cheating.
But again, that's the cruelest thing you can do. Problem is, if you really want revenge that bad, you don't have the self control to pull it off, so you'll just do the ******* thing and it'll blow up in your face sure as the sun rises. You'd be better off just telling her you know and get the hell out.