8bit killer is a lovely little retro Game Maker-made FPS that combines a NES aesthetic with gameplay and 3D engine similar to Wolfenstein 3-D. It consists of 4 chapters with 3 levels each. It's quite amusing and should be able to entertain you for an hour or so (assuming you use Windows).Download here.
Use the WASD or arrow keys to walk, press space to choose between weapons, move the mouse to look, press the left mouse button to shoot and press the right mouse button to open doors. Esc ends the game.