First-person shooters existed before Wolfenstein 3D in the same way that platformers existed before Donkey Kong. Wolf3D cemented id's reputation as the foremost developer of FPS titles, giving programming wunderkind John Carmack the opportunity to crack his knuckles and expand on Hovertank 3-D and Catacomb 3D, Wolf's predecessors.
Wolfenstein 3D, the grandfather of first-person shooters, turns 25 years old today. That silver anniversary is important not only within id's history, but the history of the industry at large. If, as a PC gamer, you look to the latest and greatest FPS to show off your new rig or upgraded parts, you've got Wolf3D to thank for setting that standard. Likewise, Wolf3D set the mold for FPS games that featured action at breakneck speeds.
Equally important, Wolfenstein 3D established shareware, a method of software publishing popularized (although not invented) by Apogee founder Scott Miller, who partnered with id Software to release 2D platforming sensation Commander Keen before the tandem made even bigger waves with Wolf3D.
If I'm being honest, Wolfenstein 3D doesn't have the same legs as Doom or Quake, its descendants. Its core, however—mowing down Nazis and introducing Adolph Hitler to the business end of your chaingun—remains a pleasant way to kill a Saturday afternoon. So why don't you do just that by using some of the mods and source ports of the game I've linked to below? And after you've done that—or before; you know, whenever—click "Visit Chatty to Join the Conversation" to share your memories of Wolfenstein 3D.
Note that you'll need the full version of Wolfenstein 3D for most of these mods to work. You can get it for cheap on Steam ($4.99) and GOG.com ($9.99, a price that includes the sequel, Spear of Destiny) among other digital retailers.
ECWolf is a source port of the original game that adds a few modern frills. Among others, you'll get modern control schemes (remember, the WASD layout didn't become boilerplate until Quake), widescreen support, and some high-resolution textures.
Wolfenstein Mission: First Encounter
This one's pretty straightforward: a 34-level expansion pack for the original Wolfenstein. Running this campaign requires the ECWolf port, linked above.
Batman vs. Bane
Back when id was an independent studio, Carmack and company made a habit of releasing their source code to the masses. That simple gesture, born of the hacker mentality on which the company was founded, enabled players to extend the game's lifeline well beyond when it should have circle-strafed its way to the grave. Batman vs. Bane, a total conversion based on the popular Knightfall story arc from the 1990s, is a prime example of how far a fan base can push old tech and still come up with fun and interesting concepts.
Hell on Wolfenstein
For players who like a little chocolate with their peanut butter. Hell on Wolfenstein integrates Doomguy and his (some of) his arsenal into Wolf3D. It's still in alpha stages, but fun to tinker with.