So I've set up a TF2 MvM server on my Mac mini. Lately, I've been playing a bit of this with my brother. It's good stuff, but requires a half-decent team to get by at the higher difficulties. We've found pub gaming to be a pile of crap, so if anyone would be keen for some robot shooting fun, let me know and we'll get stuck into it.
As you know, TF2 is free on steam, so anyone can get it and play. No Pay to Win bullshit either.
More details about MvM: http://www.teamfortress.com/mvm/