It's remarkable to think back on the joy Just Cause 2 has given me in light of its absurd sale price right now on Steam: $2.99. For that little money, it'd be a mistake not to add this to your library, plain and simple. Even if you've no intention of trying out the mod-enabled multiplayer, there's almost certainly some aspect of the game that'll click with you.
While Just Cause 2 regularly goes on sale, I don't see it getting much cheaper. The fact that Square Enix is comfortable going this low is already staggering. No more excuses! I only nag because I care and would want you to do the same for me. Friends make friends play JC2.
The video that sold me on Just Cause 2 as the definitive open-world game: