id Software

id Software is one of the most influential computer-games development company in the business.
Since the Commander Keen series, which included very popular platform computer games, they have changed the computer-games world many times:

• Wolfenstein 3D was one of the first FPS (First Person Shooter) games, and brought the popularity to the new genre, which allowed players to travel through huge 3D levels, seeing it all from the main character's own eyes.
• The DooM series (that included DooM,DooM 2, Ultimate DooM and Final DooM) were the first FPSs to use high-performance graphics engine and present a high quality of level design, comparing to other games of the time.
  Additionaly, the games brought players the option to edit game levels and create new ones, using its special .wad files system. This later developed to game mods.
• First Quake may have "just" continued the DooM series, but GLQuake was the first game to use the quality of 3D accelerators and produce gfx smoothly and quickly.
• Quake 2 brought wide multiplayer options and spread the deathmatch around the world, creating huge groups of players, playing with (but mostly, against) each other on game servers.
• Quake III Arena has the best graphics engine so far, and allows great multiplayer options, being one of the top online games (along with Unreal Tournament and Half-Life).

In addition, many developers/designers that have worked, or are still working, for id Software, are considered some of the best in their field (e.g. John Carmack, Adrian Carmack, John Romero, American McGee, Kevin Cloud, Graeme Devine, Robert Duffy, Todd Hollenshead).

Also, id Software runs the annual QuakeCon, a special conference devoted to Quake series and related games. The conference consists on a big Quake contest, presentations of quake engine-based games, and a huge LAN Party, and is considered one of the most important events of the industry.
Additional information about QuakeCon can be found at http://www.quakecon.org/

The name id doesn't stand for anything, it is called after a concept Frued used to explain part of will (Or something like that, I didn't understand that either ;).
id Software's official website explains:
Named for the instinctual part of the human psyche first identified by Freud, id’s software development team continues to make gaming history.
(http://www.idsoftware.com/business/home/history/).

id's next product is a new DooM game, and the anticipation for this title is extremely high, judging by the success of id's recent games.

Official Website: http://www.idsoftware.com

Updates:
October 11, 2001: Fixed a mistake about the company's name source. Thanks enth.
June 4, 2001: Added some hardlinks and some formation polishes..
May 3, 2001: Again, fixed mispellings, worked on the write-up's format and design, Added quote from idsoftware.com considering the name issue. Thanks randir for the comments..
April 28, 2001: Fixed some grammar mistakes (oops? ;), Added parts about id workers and QuakeCon, Added some links.

The id founders, John Carmack, John Romero, Tom Hall, and Adrian Carmack (no relation to John), were all working for a software company named Softdisk in Shreveport, Louisiana when they decided to start working on their own games on the side, and ultimately quit to found id. Their new company started in Shreveport (with their games distributed by Texas-based Apogee, whose founder Scott Miller had written some text-mode game programs for Softdisk), and moved to Wisconsin (Tom Hall's home) for a brief period before settling in Dallas (only a three hour drive from Shreveport, where they started).