Every once in a while, Extra Credits puts a video together a video about lesser known games. Quite a few of them are free, so you may want to give some of those games a look . *coughTheStanleyParablecough*.
Survivers with an "e" because it matters! But I can't play it on my laptop so I think you need a normal desktop computer for it :C Only reason how I know its good is because I've seen let's play videos on YouTube of it :I
"System Shock 2" seems to be free and community-supported. A really deep, atmospheric and long singleplayer fps-rpg. Its still pretty old, and you'll notice that in every detail, but it's worth getting through. Black Mesa, the pretty popular fan-remake of HalfLife 1 would also be a strong contender, even though I like the original more. what else... hmm... The 'free' aspect is really difficult, because while I could dig out something, it'd be most possibly a more and more obscure line of indie-games that interest me and nobody else, or some other not well aged games.