The really great thing about Arma 2's free to play version is there are no accounts to sign up for, it's just a download and you are in. The game does include some co-op and multiplayer, though there is no actual campaign available to play - it's just set scenarios. You'll also be without modding ability, but you can use user mods. You'll also be missing out on "hi definition" graphics.
Here's the bullet point list of features.
- Free military simulation – An authentic battlefield simulation, including bullet ballistics, material penetration and much more.
- Free armory – A comprehensive range of 300+ weapons, units and fully-usable armed or unarmed vehicles across air, land and water classes.
- Free unique multiplayer – Multiplayer with large-scale maps for 50+ live players and hundreds of AI units. Supports a wide variety of MP modes from co-op and PvP modes, to total mayhem!
- Free dedicated servers – Both Linux and Windows OS dedicated server support for users to host freely wherever and however they like even to play together on the same retail game-servers.
- Free creativity – Quick mission templates, a powerful, built-in, easy to use SP/MP mission editor, and advanced scripting.