Portal Mod — Unity
If you’re itching for Portal 2, this mod may be able to sate your hunger until the game is released. Portal: Unity is a mod for the popular single-player puzzle platformer that includes new textures, a new protagonist, a new antagonist, redesigned portals, and new story levels.
Levels in Unity tend to be more difficult than in vanilla Portal–the mod assumes you’ve completed the stock game and continues ramping up the challenge new with gameplay mechanics.
The custom voice acting sounds professional, but is sometimes difficult to understand–subtitles are sadly lacking.
Someone uploaded a video of an entire playthrough of Portal: Unity, which can serve either as a teaser, or as a walkthrough. Your choice.