Bohemia Interactive's long-runnning Arma series stands out from the modern military shooter crowd through its dedication to authenticity. In fact, in 2013 we named Arma 3 our Simulation of the Year, quite a feat for an FPS. If that sounds like your bag, the Humble Arma Bundle offers one of the best prices ever for Arma 3 ($15), nevermind the fact that you're getting much more at that price.
For $1, you get Arma: Cold War Assault, a re-release of the 2001 shooter Operation Flashpoint, plus Arma Gold Edition and the turn-based strategy spinoff Arma Tactics. Beating the average price adds Arma 2, the British Armed Forces, Private Military Company, and Army of the Czech Republic add-ons, and the standalone expansion Arma 2: Operation Arrowhead.
Break the magic $15 mark and you'll also get the most recent additions to the series, Arma 3 and Arma 3 Karts, which was originally an April Fools' joke but was so well-received by fans that Bohemia went ahead and made it into real DLC.
The bundle also includes a link to the free prototype for Project Argo, a 5v5 competitive tactical FPS that was announced last year. You don't actually have to buy the bundle to get access, though, you can just click here and have at it.
Arma 3 is still $40/£30/€35 on Steam (plus another two bucks if you want Karts, and why wouldn't you?), and the earlier games in the series aren't freebies either, so this is a pretty solid deal if you're at all interested in giving the series a go. The Humble Arma Bundle is live now and runs until March 14.