Bohemia Interactive has released Arma 3 Alpha Lite, a free limited version of Arma 3 Alpha which currently is amongst the top sellers on Steam.
The only way to get the Alpha Lite version is to receive an invitation from someone who has purchased a copy of Alpha. Bohemia Interactive will also be randomly distributing invitational codes through their Arma 3 social networks.
The free Lite version will not include multiplayer or any sort of modding support. The notion is to provide people with the opportunity of trying out the single-player capabilities, with a selection of missions, vehicles, weapons and the mission editor.
Also note that being invited to Alpha Lite does not guarantee you a spot in the beta nor gives you access to the full game on release. The Arma 3 Alpha Lite will be discontinued on June 18th, 2013.
The full game is expected to be released in Q3 of this year.