It's DOOM.
This is DOOM!
I was first introduced to DOOM in the mid 1990s by a friend in elementary school, who had let me borrow a shareware copy of it that was on floppy disk. That's right, a 3 1/2" floppy disk. As soon as I got home from school, I popped it in the family 486 computer (which had recently gotten a Sound Blaster 16 sound card and CD-ROM drive added to it).
For a six year old, it was like finding your dad's (or older brother) stash of 'gentlemen's' magazines. It was bloody, it was violent and it was fun. My only regret was that we didn't have a modem. (We didn't get Internet until around 2001). As it was shareware, I only had episode 1. The full game was three episodes (four if you got The Ultimate DOOM, which was the retail version of the game).
The prospect of getting it was null and void though, as my parents weren't going to drop the money for it on a kid my age. So I was stuck with the shareware version until the disk died.
Fast forward to 2004. Fate had deemed that I would be reunited with DOOM as I looked through the budget titles at the local Wal-Mart in Van Wert. And there it was. DOOM. DOOM II and Final DOOM. All on one CD with an extra CD touting DOOM 3 preview material for 10 dollars. I bought it, brought it home and set it up. A massive DOOM blitz followed, and I was talking it up to my bewildered classmates like I was a teenager back in 1993.
DOOM takes place across three episodes (four if you have The Ultimate DOOM). The first two take place on Mars' moons, Phobos and Deimos. The third and fourth take place in Hell. The whole story is that you're a disgraced private soldier, working for the Union Aerospace Corporation, which is working on teleportation technology on Mars. Their work has caused a portal to Hell to open, prompting hordes of Hell spawn to emerge through and take over Mars' moons (and most likely Mars itself as well).
Sent to contain the demons on Phobos, your squad was massacred and being posted at the opening of the hangar, you are 'lucky' enough to be alive. With 9 levels to go through per episode (the 9th being a secret one accessed at each episode through different ways), you must simply blast through the demons, collect keys and reach an exit button located somewhere in the level. At the 8th level, there will be a boss demon to confront.
DOOM features five difficulty levels:
I'm Too Young To Die (You wimp)
Hey, Not Too Rough (C'mon..)
Hurt Me Plenty (We're getting closer)
Ultra-Violence (You should be playing on this difficulty)
NIGHTMARE! (You should be playing this difficulty if you want an extra challenge to Ultra-Violence: Respawning and faster demons)
As far as weapons go, you start off an episode with just a pistol and your fists. You can work your way up to a single barrel shotgun (my preferred weapon of choice for taking out the rank and file mooks), a chaingun, a rocket launcher, plasma rifle and the BFG (known lovingly among DOOM fans as the Big F-cking Gun). You may also find a chainsaw that can be alternated with your fists.
Of course while single player is just plain awesome, the main reason DOOM got so much hype is because of its multiplayer offering. You could play through the entire game cooperatively or frag your friends back in 1993 through a modem, serial link or an IPX network, with the only drawback being a restrictively low player cap. Thanks to the evolution of the Internet, you can still play it today over the Internet with a larger player cap with the TCP/IP protocol through programs called ports. The main Internet ports of choice are ZDaemon and Skulltag, though you can find more out there by searching.
DOOM is also touted for its openness towards modifications. A horde of modding tools and mods have come out since DOOM's release, with books and online instructions to match. I myself am a proud owner of the classic Hackers Guide to DOOM by Hank Leukart, which covers the mainstream DOS based tools (such as DEU). Some hardcore modders still use these legacy tools, but for the greenhorn, there's nice Windows applications like DOOM Builder.
If you come across it, buy it. You won't be sorry. You may luck out like I did and find DOOM Collector's Edition at a store. But if you prefer digital downloads, Steam offers The Ultimate DOOM itself for $9.99. Or you can get the entire series for $40. (I see it as a bit of a ripoff though compared to the DOOM Collector's Edition. 3 games = $10 compared to 3 games = $29.97) BUY THIS DAMN GAME!