The Saxxy Awards, Valve's annual Source Filmmaker competition, is now in its fourth year, and to shake things up a bit they're encouraging (opens in new tab) players/directors to make films about something other than Team Fortress 2 - namely Portal. All of Portal 2's assets (minus, er, the portals and a few other things) can now be downloaded into SFM as free DLC, enabling that Spy vs Wheatley crossover animation that (probably) hasn't been able to exist until now. That isn't the only content pack headed to the program either: Puny Human are offering up select assets from Blade Symphony too.
The deadline for submissions this year is September 24th, giving you just under two months to create your masterpiece. You'll find the guidelines for the awards here (opens in new tab) .
If you need inspiration and/or films to compare your efforts to, be sure to check out the winners (opens in new tab) from last year. This one won best short: