Portal: Alive & Kicking was announced a few days back. Its a Portal 2 addon, which will recreate the adventure of Chell during the adventure of the original Portal, with the current look of Aperture labs. So why do this is a few other people are doing it too, and why play it?
Portal: Alive & Kicking is pretty much a collaboration of recreated Portal maps for Portal 2. Not only Colossal and I are working on it, but also PortalPlayer, an admin for myApertureLabs, Omnicoder, mapper of the third place entry Edifice of the Summer Mapping Initiative Contest, Groxmapper, and Wauterboi, who does music.
This project is still under development, so please watch the ModDB page for more updates.