Currently free on humble store, but giveaway should start right as the promotion is over.

This one is just for the ones that missed the free copy over at humble store, as I already own the game.

Immediately after 10AM Pacific on April 14th, the Steam key will magically vanish as if it were just a figment of your imagination, leaving this cryptic message behind in its wake.

I don't know if that means the key will be revoked, but just make sure you activate it before that time.