Doom 2

The quintessential Steam release of Doom 2. A great port of an all-time classic.

 

This game is literally just Doom 2 playing on an instance of DOSBox, but it’s a classic for a reason. 
You already know the story: demonic invasion, loads of bad guys to shoot, and one doomed space marine – a second time. There may be almost twelve different colours in the palette and four notes in the soundtrack, but it holds up in every area where it matters gameplay-wise. Don’t expect out of the box mouselook support or handholding. 
Just get it, install it in three seconds flat, and jump in already.