Portal: Prelude, a free, unofficial Portal mod with prequel storyline
“At this time, test subjects were monitored by real Aperture Science employees whose work was tedious, lengthy and repetitive. This is why they decided to build a great artificial intelligence that could both replace them in these difficult tasks [...] and compete with Black Mesa’s superiority. All employees of the Aperture Science complex are now eagerly awaiting GlaDOS. Maybe even a little too eagerly, as the upcoming events will tell…”
Reviewed by Gamespy.