Although Rising Storm Game of the Year Edition normally costs $19.99, Humble Bundle is currently giving copies away at no cost. Host on the Humble Store, checkout requires a login and nothing else. Though, you will be asked to donate to charities that help a variety of causes, but this isn't mandatory.
It is unclear how long this promotion will last. It appears to be a marketing campaign for Rising Storm 2: Vietnam, which releases early next week. This means there are no strings attached, and you might only be able to take advantage of the deal for a few days.
Note that the code you receive will unlock the game on Steam.
Rising Storm earned accolades when it released in 2013, going as far as winning multiplayer-oriented game of the year awards from several outlets. It's made by the same team behind the popular Red Orchestra series, Tripwire Interactive, a franchise that aims for realistic battle scenarios based in historically significant eras of war.