Beyond Black Mesa is a live action short based on Valve's Half-Life 2. It is based in the world, but there i no Gordon Freeman or ALyx Vance, instead it follows Adrian Shepard and his compatriots, somewhere around Ravenholm, action as a resistance force trying to get a transmission out with the Combine hot on their heels. It's a pretty high production independent film. There are enough subtle reference s to the original game throughout to keep fans happy and they never interfere.
A total of seven friends managed to put this together over a period of 2 years for about twelve hundred dollars. Now that is impressive. Check out the movie's site for more information.