To answer this specific question - AdWords advertisers have a setting in their Content Network campaigns called "frequency capping" where they can set the maximum number of times an ad is shown to a unique user. I've never used it myself. You can set the number of impressions the ad gets, and also the time period over which they get them (day/week/month).
That setting seemed to just appear with very little fanfare; I'd be surprised if most advertisers even knew it was there.
It'd be kind of nice if we publishers had something similar, but I don't see it happening any time soon.