Both as a test of their skills and an exercise in fanservice, a small group called Zombie Games has recreated an entire level of PS2 shooter Killzone in the Source Engine on PC.
The folks at /v/ from 4chan headed over to Valve to wish Gabe Newell a happy birthday (in a pretty intricate and ingenius manner) but in the fracas some questions were asked, and it looks like Valve could actually be working on a new Source engine.