The Jacknife Chronicles is a remarkable short film made entirely in Garry's mod. It's so good it made me say "woah" out loud, and then "what the-?", then "did he just-?", and then I said nothing, because my mouth was hanging open.
It's a revenge thriller set in a really, really dark version of Half Life's universe. Something very blurry and bad has happened and a man in a white shirt needs to die. Cue 12 minutes of explosive, first person hyperviolence in which our man parkours his way into the centre of a facility, passing through areas from Mirror's Edge and Portal as he goes. Check it out.