If you have pre-ordered Arma 3 then you already have access to the alpha version of this hard-core military simulation. If you haven't already purchased your copy, you now have a shot to check out Arma 3. A free lite version of the alpha is out on Steam that doesn't include mod support or multiplayer.
This free alpha is invite-only, and people who have pre-ordered are getting keys as gift items in their Steam inventory to give out as they see fit. Bohemia Interactive will also be giving out keys on Arma 3's Facebook and Twitter. I'm looking forward to taking a look to see what's different from Arma 2.