Portal: Prelude

Portal: Prelude is an unofficial prequel to the game Portal, developed by an independent team of three. The game features a brand new storyline focusing on the pre-GLaDOS era of Aperture Science where test subjects are monitored by employees of the facility. The game spans across 8 chapters covering 19 test chambers which follow a gradual learning curve similar to that of Portal. In addition, 48 challenge maps and 6 advanced test chambers are included with the mod.

Many gameplay elements from Portal make a return in Prelude, while entirely new gameplay devices were created exclusively for the mod, such as localised gravitational fields. Portal: Prelude also features over 400 lines of text to speech, with English, French and German subtitles.

Awards

 * Portal: Prelude won third place in ModDB's Mod of the Year 2008.

Trivia
Though other female characters are mentioned, all of the employees the player sees in the mod are men and have nearly the exact same design, excluding the party hats near the end of the game. The only exception is an unnamed female employee wearing a face mask seen dragging Abby out of GLaDOS's room during the ending cutscene.

The G-Man from the Half-Life series is also seen many times in this mod, possibly to warn Aperture Science of "unforeseen consequences".