In the greatest music/game tie-in since Marky Mark asked you to make his video, Valve has announced a music video contest for The National's song "Exile Vilify" (featured in Portal 2). Though the song was written for Chell's second trek through the Aperture testing facility, your music video doesn't have to be machinima -- Valve says creators can use "whatever visuals they choose."
The deadline is July 15. Once you've pieced together your masterpiece, you can post it to YouTube using the tag "PORTAL2NATIONALEXILE" to mark it for the contest. The announcement promises that Valve, The National, and the community will select a winner, implying that there may be a voting aspect to the proceedings.
The winner will get a guitar autographed by The National and some Portal 2 merchandise. Valve is a little vague on what exactly that merchandise will be, but the company store has no shortage of items that could be included in the prize.