Portal Easter Egg - Black Mesa in Portal

During the final boss battle with GLaDOS, many pictures will flash across the screens, most too fast to really see. Some of these are "subliminal messages" with EXIT signs and WARNING signs. However, there is also included a nice picture of the Black Mesa logo.

To find it, get to the boss battle chamber and sit somewhere where you can see the screens. Do this before you destroy the morality core, because it may take a while. Keep pausing, unpausing, and pausing again, really quickly. This will allow you to view about every other picture at your leisure. Eventually the logo for Black Mesa (the laboratory from Half Life which Aperture Science has a huge rivalry with) will be shown.