CryEngine available on Steam

[Memnoch]

You too could write the very next, latest and greatest sci-fi opera.

http://store.steampowered.com/app/220980/

With a number of subscription plans costing as little as £5 a month, it won't exactly break the bank either.

 

[Toumal]

Note that this is not a source license. You only get the binary build and the C++/scripting interface together with a small sample game. Which is fine if you're doing a fishbowl shooter or something similar, but if you want to do something that goes beyond what the interface provides, you can't do it.

That said, it sure beats Unity for PC