The most common way to get a legitimate free copy is through the . They have featured Salt and Sanctuary as one of their "Free Mystery Games" in the past.
Add it to your Steam Wishlist to get an email notification the moment the price drops. Salt and Sanctuary Free Download
It's important to clarify that is a commercial indie game, and there is no official "free" version provided by the developers (Ska Studios). The most common way to get a legitimate