Free 3D game in first person, like Quake or Doom, with great graphics and atmosphere. For this the graphics engine works like OpenGL or SDL, and as a result gets the light just right, with good visual effects (shots, shade, explosions...).
The game mode can be with one player, where you fight on different levels against monsters controlled by the PC, or in multiplayer, where this game becomes popular. It looks for servers automatically, which is useful for you to find other players on the Net, although you can also play on a local network.
The icing on the cake is the map editor, making the fun last longer.