Spore

It's the new 'coolest thing ever' and it's called Spore. I'd heard about it before but never really got excited about Will Wright's latest creation until I saw the lecture he gave at the 2005 Game Developers Conference. The whole concept is simple and simply amazing, and the programming behind it all is completely mind-blowing. I won't bother trying to describe it in detail - just check out the edited video and see for yourself.