It’s pretty simple to do and legal. From now till December 16 you can get a copy on Steam for free and all you have to do is like their facebook page in order to get it.
It will be in limited numbers so you better pick it up as fast as you can. The downside is that its only on PC and you need a stream account but Steam is free. if you dont know anything about Metro 2033 here is some gameplay of the free PC one that you can pick up