"Now you're thinking in portals"

The upcoming Half-Life 2 episode is probably going to be pretty amazing. I haven't finished HL yet and I haven't even played HL2, but the upcoming episode is supposed to have a really cool new single-player fps which will turn the gaming industry on its ear. It's called Portal and just looks awesome. It's built on the HL2 engine called Source (warning: loads of big words on that link) - all of which has been written by Valve. Check out the demo video which is mocked up to appear like a training video for a new employee. It's impressive to say the least.