It feels silly to tell you about a game that's going to be free months from now, but maybe, just this once, it'll prevent someone from buying said game right before the promotion begins. With that in mind, Payday: The Heist will be free for 24 hours on October 18, 2014.
This offer is connected to Overkill Software's Crimefest, a celebration of the Payday franchise's third anniversary happening in October. The studio has a goal of 1.5 million community member sign-ups on the Payday 2 Steam group, and there are milestones with rewards. The group now has over 1,050,000 members, hence the free Payday offer.
Even if you're done with the original game, I'd encourage Payday 2 players to take a look at that link -- the other rewards will be relevant to your interests. Looks like we'll get them all, too.