NZGamer writes: "Are you new to the whole Portal phenomenon? Then stop reading this and go buy Portal: Still Alive right now. It's safe to say it's the definitive version, and will only set you back 1200 Microsoft Points.
If you're still not convinced, or have already played through Portal, then by all means read on.
A quick sum up: Portal is a first person puzzle game. You're trapped in a sterile laboratory, and must navigate through a series of mind-screwing puzzles using the titular portal gun. This device lets you attach portals to walls, through which you can then walk through. At a basic level, imagine coming across a chasm, then creating two doors on either side of it. You can then walk through, and you're safely over the other side."