GMan easter eggs in Half-Life 2 games.
Today I was play Episode 2 where you're ambushed at the abandoned town, but before the spot you're ambushed you see GMan walking between the houses.
Or when playing an awesome mod, Portal Prelude. In the end you have to find a portal gun, so you end up having to go back toa window where you had to give it up. In the room, if you look out the window, you see GMan walk down the hall.
Again in Prelude, after you finish Test Chamber 19 and enter the lobby, GMan is walking on the balcony.