Shooters published under the 3D Realms banner always tried to push the boundaries -- be they technical or moral -- and 1997's Shadow Warrior is no exception. It hasn't aged terribly well, but if you're a fan of titles such as Duke Nukem 3D and Blood, then chances are a free copy of Shadow Warrior will tickle your fancy as well.
Publisher Devolver Digital recently dropped the price to zero on Steam, so anyone with an account on Valve's distribution service can snag themselves a copy right now. Going by the system specs on the product page, it's not just Windows users who can benefit -- the game now appears compatible with OS X too.
Along with Duke and Doom, Shadow Warrior was one of those games that got a regular workout at LANs I attended in my younger days. I remember being impressed with the reflective floors, until I learned later that all the level designers did was mirror the room and place it below a slightly transparent floor.
Now, that was the frontier of 3D.
Shadow Warrior Classic [Steam]