I tend to get a little upset (not necessarily ripping mad) and then try to not put myself in the same position again. I forgive, but don't forget.
For instance: BIL called me up a couple days after Thanksgiving claiming that I'd said something to SIL that offended her on TG. It didn't happen... just plain didn't go down the way he said at all. BIL said, "If you won't sit down and have a meeting to discuss the lies you're telling, then I don't want to ever have anything to do with you again." Now, I'm generally a nice person and if I inadvertantly said something that could be remotely considered offensive, I'd be the first person to offer an apology. But I'm not going to roll over when I've done nothing wrong. And I'm not going to put myself in a situation where I'm going to be sandbagged with unfounded accusations. BIL and SIL live 5 doors down from us and I had considered SIL a good friend. Now, I have nothing against BIL and SIL. DH and I would be happy to see them at family events (they are avoiding any Christmas celebrations with family at which we might be present). But I'm not going to be friends with SIL, anymore. There's no need for me to waste my energy trying to "get even" but also no need to put myself in a situation where I can be blamed for BIL and SIL's issues again in the future.