Mega Man Quadruple Time Attack (2007)

A tool-assisted speed-run in which four different Mega Man games (3, 4, 5, and 6) are completed simultaneously using a single set of button commands.
 

For answers to questions like why the four games don't appear to be in synch with each other, or how something like this is even possible to begin with (hint for both: lag exploits), check out the authors' extensive commentary here.