Steam has a peculiar history with the word "free", thanks to its regular Free Weekends. Through them, you can get a free trial of the entirety of a game across a limited two-day period. Red Orchestra 2 will soon have one of these weekends, and an overall discount to go along with it. Even freer, though, is a deal that will go live for the multiplayer shooter later today. For a 24-hour period, you'll be able to download (and keep) the game forever, for free.
"Yes, Free!" confirms the enthusiastic Jared Creasy, Tripwire's community manager. "By navigating to http://store.steampowered.com/app/35450/ and downloading the game tomorrow after the promotion starts and before it ends (the promotion will last 24 hours), the game will be free to keep forever!"
It's not entirely clear when the deal will go live, but I suspect it'll be at the start of the new Steam-day—around 10am PDT, or 6pm BST.
While it is a great deal, it's worth remembering that through Red Orchestra 2 you only get limited "rifle only" access to the Pacific-set follow up, Rising Storm. Owners of Rising Storm—which recently updated with a free Game of the Year edition—get full access to RO2's multiplayer.