I received an email from an AdSense competitor announcing a new type of ad. If the ad unit is designed to typically display 4 ads, yet 7 ads (for example) are available, it will display all 7 ads in the same ad unit space.
Their research has shown this can increase CTR. One example is showing 6 ads in a unit vs. 3 ads in the same unit resulted in a 47% CTR increase during their testing.
I'm going to be doing some testing of my own.