The unofficial mod
http://www.portalprelude.com/
Well, needless to say it is portal...
Example chambers (by me)Portal: Prelude, as its name states it, is an unofficial prequel to the game Portal. Its story revolves around the pre-GlaDOS epoch, even before she was plugged in. 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, but also take responsibility for many other tasks within the complex 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...
May be spoilers
Chamber 5: http://www.youtube.com/watch?v=M01G1eYMDCo
Chamber 6: http://www.youtube.com/watch?v=w13nt0BYNpg
(available in higher quality)
So far so good, has the same kind of dry humour e.g. at the start of one level, a block falls on your head to the erupting laughter of the Aperture Science employees.
Nice, clean and easy music scores keep it flowing, with a few twists in levels that ensure it takes you a few attempts to get through it right.
So for an initial opinion it seems to tick all of the good boxes, has enough substance to make you want to keep playing, but not too much that it just drowns you in puzzle hell.
The only niggle i have with it at the moment is the (what i find annoying) voices, as they have the tone change thing mid sentence, which makes everything sound over synthesised... Which as it is pre GLaDOS i would have expected fluid voice casting (i.e. pre GLaDOS therefore with real people monitoring the test subject), it's not a big issue, but it's one that i think could hold it back slightly, as i believe with the voice casting, it would add a deeper element and make it a more immersive atmosphere...
Updates as i get further into it.