Few games revolutionized a genre as much as Doom did with first-person shooters in 1993. This is underlined by the fact, that even today there are clones based on the GPLed engine under active development, the most prominent being PrBoom. The game is usually being distributed together with FreeDoom, which contains all the graphics and levels necessary for actually playing the game.
PrBoom is an "old school" first-person shooter, meaning that rather than hunting computer-controlled bots there are dozens of levels filled with monsters. In order to complete a level you only have to reach the exit, but that sounds easier than it usually is.
The game offers five difficulty levels, ranging from "I'm too young to die" to "Nightmare". Higher difficulty means more monsters and less ammo, medkits and armor lying around. As the labels indicate, there should be a difficulty suited to everyone's ability.
Several monsters attacking
When you play PrBoom for the first time, you will immediately realize that the foundations of the rendering engine date back to the early nineties. Although the rendering is done via OpenGL and the resolution has been pushed up from the orginal 320x200, the graphical effects look rather poor from today's point of view. Another problem is that the engine does not deliver a full 3D experience, possibly bringing back the long forgotten game sickness. An obvious boon of the antique engine, however, is its ability to run fast on almost any machine.
Playing is a little bit different to modern shooters as you cannot look up or down. Aiming in the vertical direction is done automatically, which means that you can play the game by using only the keyboard. This might be handy if you have a notebook and want to play while waiting for a connecting flight, for example.
PrBoom also offers a two multiplayer modes, one being the usual deathmatch, where the players try to kill each other. However, single-player maps are not very suited for this playing mode. The other option is to play the single-player levels cooperatively and fight together against the swarm of monsters.
Although the game is not too bad, you should only care about it if you are a true Doom enthusiast or have a really slow computer. Nowadays, there are much better open source shooters out there.