Above are three CG references you will come across most frequently while watching SIGGRAPH demos or any other research papers.
You can read more about each of them in the links above but in essence, all of them were created by a graphics researcher(or group of them) to test & demonstrate CG techniques/algorithms.
P.S. : Lenna is the 2d image counterpart of above 3d objects & is used in numerous image processing & compression demos. And has most sexy(literally) history as the 512 x 512 image came from a scan of November 1972 issue of Playboy.