Portal Easter Egg - Black Mesa Competition

When in the underbelly, after walking across the long bridge, you come to an office-type hallway. You are given the option to go left or right. Going left takes you to the hallway to GladOS but if you turn right, down the shorter hall, you come to an un-openable door with a window in the adjoining wall. If you put your back to the door and look into the window, after a moment of adjustment you can see a short presentation detailing financial competition between Aperture science and Black Mes.
The joke being that Valve created Portal using the same revolutionary shooter technology developed for HalfLife.