Being an alpha release hasn't stopped Arma 3 from scouting out the top end of the Steam sales chart. It's probably because of the hyper-realistic military simulation scenarios that the game offers. But if tactically significant turtles aren't enough to persuade you to part money at this early stage of development, Bohemia have released a free Lite version. It does, however, come with some caveats.
The big one being that it's invite-only. To access the Lite version you'll need a key from someone who's purchased the Alpha. Of course, if any particularly charitable Alpha owners are at a loss for where to send their pass, I'm sure you'd find some willing takers in the comments.
With the Lite version you're only getting a small taste of the full alpha package. Specifically, multiplayer and modding support have been stripped out. What's left are the singleplayer showcase missions - giving you a chance to play with the game's infantry and vehicles. And go scuba diving.
For everything else, you'll need to upgrade, which will also provide access to the eventual beta and full release.