As usual, I'm only posting this because I've personally done business with Humble Bundle and they are absolutely legitimate. There's no gimmicks, no payment information to enter, no ads to waste time sitting through; it's a game for free and that's it. All that's required is you sign up for a free account with Humble Bundle, which in my opinion is worth it's weight in gold. Portions of your cash purchases go towards charity which is so great, plus the deals they come up with are some of the best on the interwebs.
According to Humble Bundle's synopsis of Outlast, "In the remote mountains of Colorado, horrors wait inside Mount Massive Asylum. A long-abandoned home for the mentally ill, recently re-opened by the 'research and charity' branch of the transnational Murkoff Corporation, the asylum has been operating in strict secrecy…until now. Acting on a tip from an anonymous source, independent journalist Miles Upshur breaks into the facility, and what he discovers walks a terrifying line between science and religion, nature and something else entirely. Once inside, his only hope of escape lies with the terrible truth at the heart of Mount Massive. Outlast is a true survival horror experience which aims to show that the most terrifying monsters of all come from the human mind." Spooky, and just in time for Halloween! As always, happy gaming!