OCP

This Company is Originally from the Film Robocop. On the companies Remote Monitoring System, you can find the 4 Primary objectives of Robocop. The 4th is classified, as it is in the film. It states that Robocop could not harm anyone Executive under the employ of OCP. There is nothing you can actually do on this server in the game, although sometimes the company will hire you to do missions like every other company in the game.
Wikipedia Entry: Omni Consumer Products

Steve Jackson Games

Steve Jackson Games is a company that make many types of games, including GURPS and Card games. On the 1st March in 1990, the company was raided by the secret service. They seemed to consider one of the games made by Steve Jackson Games, GURPS Cyberpunk, to be "A handbook for computer crime" despite the fact that it is set in the future.
There is nothing on the server in the game, except some text that says the system has been saved and you IP has been logged (don't worry, they don't really log your IP and nothing happens from it). Like Every other company in the game, they will some times hire you for missions. More info can be found at http://www.sjgames.com/SS/
Weikipedia Entry: Steve Jackson Games

Protovision

Protovision is the Company from the 80's movie 'Wargames'. The movie was about a kid who thought he was playing a game but accidentally triggered a countdown on a computer system to start world war 3.
On the server, you can play a small game which involves clicking on cities to fire missiles at them (that's about it really) but first, you have to find the password...
Wikipedia Entry: WarGames

Introversion Software

The makers of Uplink, this server is included in the game to contain information and ideas for a multi-player Uplink. There is no hacking to be done on the site, though you do have to find the IP address first.
You can find Introversion at http://www.introversion.co.uk
Wikipedia Entry: Introversion Software

ARC

The 'Bad Guys' of the game, ARC are involved in more than suspicious activities... Rumoured to be developing some kind of weapon, using skilled Uplink Agents to do it. In the game you will have to opportunity to work for ARC as part of the story line, helping the company perfect their new Software.

Arunmor

The 'Good Guys' of the game, and ARCs first target... They take it upon them selves to stop what ever it is ARC is planning. You will have the opportunity to work for Arunmor as part of the story line, helping them stop ARC from releasing their new software...