I got shadow warrior for $20 bucks at gamestop. It's a fun game,that feels like an oldschool fps,although using the sword is way more fun. I'm only a few chapters in and I really like the oldschool feel of the game. The graphics aren't going to blow anybody away and it seems to be using the same 3 enviroments so far,but the combat and retro feel of the game,really makes up for all of that. It's nothing fancy,but just a straight foward budget title and I really enjoy it so far. What hidden gems have you found? what other retro oldschool games are there. I'm kind of sad that this game got lost in the shuffle,but I don't mind playing bargin bin games,as long as I enjoy them. 

I thought seriously about buying it on PSN when it was on sale for a song a few months ago. I love the throwback first-person shooters where the hero carries a truckload of ridiculous weapons and takes on armies of enemies.

You might also like Painkiller or Serious Sam.

Haven't played serious sam but I thought Painkiller was pretty fun :P I got one of them for like a dollar on sale on steam once 

Just watched the trailer and it had the Transformers movie theme song on it ("You got the Touch"). So it gets extra bonus points and I might grab it when I get a PS4. Gameplay looks fun and I know the original was a classic.

It's not clear to me what makes the game "retro" though? The graphics aren't old school. Is it the tone of the game? To me, it just looks like a regular, current (or maybe last) generation FPS. I guess when you said retro/oldschool, I was thinking graphics like Hotline Miami or something. lol

Duke Nukem 3d Megaton Edition is a nice romp down memory lane and old school, for sure.

It's also in the Humble Indie Bundle 14 in the $10 tier: https://www.humblebundle.com/

So 10 bucks for Shadow Warrior S.E., Outlast, Torchlight 2, La-Mulana and 6 other games.

Absolutely loved this game. I am considering going for the Platinum trophy in it, which is something I very rarely actually care about. Only been maybe 2-3 games I have gone out of my way to do it.  I was actually really surprised at how into the story I got even though it was a limited plot.  The new PSPlus game Tower of Guns is very old school and pretty cool. It reminds me a lot of Quake 3. Not even close to as good but has similarities for sure.

If you are look for other hidden gems, I would recommend contrast. It is a short puzzle platformer which is unique. There were several bugs at the launch Contrast which negatively impacted its initial reviews. While it is not an FPS, I think you might enjoy it.