Tetris for DOS. The RAM resident version. Terminate Stay Resident, or TSRs as we used to call them.
I'd start printing a long report from dBASE II on our brand new Zenith 80286 at work, then call up the TSR. The printing would pause and Tetris comes up on the screen. I'd play for hours. When I heard the clicking of the cipher lock on our door, meaning the boss was coming, I'd hit the ESC key, Tetris would hide, and the printer would pick up where it left off. Man, I always looked busy. When the boss left the room, I'd hit the hotkey that would pause everything and bring up Tetris right where I left off. It took months before our boss figured out what we were doing.