Portal: Prelude, as its name implies, is an unofficial prequel to the game Portal. Its story revolves around the pre-GlaDOS epoch way before she was plugged in. Back then, test subjects were still monitored by real Aperture Science employees, a tedious, dull, and repetitive job. One of the reasons why they decided to build a great artificial intelligence that could both replace them for these complex tasks and also take responsibility for many other functions within the complex while competing with Black Mesa's superiority. All employees of the Aperture Science complex are now eagerly awaiting GlaDOS. A little too eagerly maybe, as the unfolding events will tell... This game is totally free. It offers a bit more gameplay hours than the original Portal, with 8 chapters, 19 test chambers, 48 challenges, 6 advanced maps, a brand new storyline and more than 400 lines of English dialogue with dozens of subtitles languages.
Pretty much a very high quality mod, it fits right into the Portal series storyline.
At some periods in the mod, I thought this was almost Valve-quality.
The only two cons I have found is:
A very monotone and boring voice-acting.
Some puzzles require a weird way of controlling the character, this could have been polished better.
Other than that, everything is as good as it gets for mods!