Half-Life: Uplink is the Half-Life title that you have likely never heard of.
Half-Life: Uplink, released February 12, 1999, is one of two demos released by Valve for the original Half-Life video game. The first was Half-Life: Day One and contained the first fifth of the full game, and was only included with certain video cards of the time. Half-Life: Uplink, on the other hand, was a standalone demo set in the Half-Life universe, and contained levels and a storyline not found in the full title.
Sadly, for whatever reason, Valve seems to have forgotten about Half-Life: Uplink. It is not mentioned anywhere on Valve's web site and is the only Valve title not offered in the "Valve Complete Pack" on Steam. One possible explanation is confusion with Uplink, an unrelated title by Introversion Software that was offered through Steam early in Steam's existence, but this is unlikely. Technical issues would not prevent it from being offered on Steam, as HL!UL!SL, a third party project, integrates Half-Life: Uplink into Steam as a Half-Life mod.
While Half-Life: Uplink is a rather short demo, it is still a unique story, and is sad to fade into obscurity. This site attempts to keep Half-Life: Uplink in public memory.
Half-Life: Uplink was originally available for download from Valve/Sierra, and was also included as part of Half-Life: Further Data V.1. This standalone disc was available for free in retail outlets, and included Half-Life: Uplink, as well as extra maps and an update patch for Half-Life itself.
Today, if you want to try Half-Life: Uplink with the least amount of hassle, try HL!UL!SL, which installs Half-Life: Uplink as a Half-Life mod in Steam. Disclaimer: HL!UL!SL is packaged/maintained by the maintainer of this web site, so there's probably some bias there.
The original Half-Life: Uplink installer is still available to download. This installer does not require Steam or a Half-Life installation, but does tend to have compatibility difficulties with modern versions of Windows.
Before the game properly starts, captions appear on-screen stating that you are playing the character of Gordon Freeman. Little other information on Gordon is alluded to, save for the fact that Gordon has a Ph.D. The captions then go on to say that he is located within the Lambda Reactor Complex, Black Mesa Facility. The time is stated as being 48 hours after the 'containment failure', and that his status is 'evaluation in progress' -- likely a reference to his recruitment by the mysterious G-Man, but can also refer to the fact that, as a player being introduced to this demonstration, you are judging its capabilities, and deciding whether you should buy the full game.
The screen fades into a corridor and suddenly, jet fighters can be heard overhead. Then, the sound of a bomb being dropped is proceeded by an explosion in the corridor, which causes some of the electronic appliances on the walls to explode, and some of the ceiling collapses. The player then proceeds forward into a large room, and comes across a scientist, and the player is then quickly joined by a security guard. The security guard asks the scientist, in an abrupt tone, if he can open the heavy metal doors which are located within the room (these doors presumably lead to an exit out of the facility). However, the scientist says that a message must be sent to the United States Nuclear Regulatory Commission (USNRC), to say that the coast is clear, and that cannot be done because a radio antenna must be aligned, manually, before the message can be sent. However, the area is "swamped with radiation", thus making it impossible for either the scientist or the security guard to do the job. However, Gordon Freeman, is wearing the Hazardous Environment Suit, and this will protect him from radiation. The scientist then goes to activate the lift which is in the room, and informs the player that he will offer directions from over the intercom. The player then enters the lift, and proceeds to a higher level.
When the player exits the lift, he comes across a corridor which has been blocked by crates and other wooden boxes on one end. In a room, nevertheless, on the other end, a crowbar has been left on the table. The player picks up the crowbar and can either use it to smash the crates apart so that he can proceed or can destroy the vending machine with the crowbar. Behind the vending machine there is a vent leading to a room with a shotgun, a pistol and two headcrabs. After that, the player reach a security booth and encounters some more headcrabs. With the assistance of an armed security guard around the corner, the headcrabs are killed. Opposite to the security guard there is a room where the player can pick up a Glock 17 pistol. The player then travels down a corridor, where he encounters more headcrabs, and also comes across some tripmines, which have been placed to prevent him from entering through one of the doors into a warehouse. Once the mines are destroyed, the player picks up an MP5 submachine gun and proceeds to fight three HECU soldiers in the warehouse, but the player can't kill one of the them before the soldier closes a barrier, which leads from the warehouse to the surface storage area outside. However, by negotiating some of the overhanging crates, Freeman is able to jump over the barrier.
The player can see that it is nighttime, and Gordon Freeman must proceed through the storage area. Freeman will come across a machine gun nest, which can then be used to eliminate several soldiers. He then comes across a scene that will be repeated several times in Half-Life; soldiers have been lining scientists along the wall and shooting them. One, in fact, will try to escape when he sees Gordon. Before proceeding through a door at the bottom of the dish tower, the player can see that soldiers have been burning the corpses of Vortigaunts, as well as radioactive sludge that has been leaking throughout in the dish tower. After entering a lift and traveling up to the antenna room, the radio antenna needs to be turned manually with a valve. The first scientist speaks on the intercom, telling the player to move it to the position marked 'USNRC'. When Gordon looks at the control panel with the valve, he sees a screen saying that the remote override has indeed been locked out. When Gordon has the antenna in position, he pushes the button, and the transmission is sent. The scientist tells him to get back, and that they'll be waiting for him.
Shortly after, the ceiling explodes and soldiers rush into the room. The player is required to get to the lift and make his way back. However, when he reaches the bottom of the dish tower, he sees that the entrance he went through has been blocked by explosives, which cannot be detonated safely. The player then has to go through a vent in the floor, and navigate his way through the complex system underneath the tower. Afterwards, Gordon finds himself back in the storage area, where he must again use the machine gun nest to destroy several Vortigaunts. Doubling back on himself into the warehouse, he encounters a battle between soldiers and Vortigaunts. The player is advised to run through, avoiding the crossfire, and to continue back towards the meeting point.
The player comes back, and can now proceed through the heavy metal doors. The player does not have to escort the security guard and the scientist, but he will find that, if he does, they will not be able to proceed past some over-turned filing cabinets (this can be avoided by killing the zombies behind them with a grenade). As the player continues to proceed down the corridor, which presumably leads to the exit, clamorous roars are heard. Continuing onward, the rumbling gradually becomes more violent, eventually causing the vent to collapse. The player finds himself in a room with two scientists, an armed security guard, and a Gargantua, with the G-Man in a room above looking through a window. The Gargantua, impossible to kill with the weapons that the player has, kills the scientists and the security guard, then turns on the helpless player. As the creature approaches, the screen begins to fade. (The Gargantua can be killed if the player has full ammo in the grenade arsenal, combining with the damage the Gargantua receives from destroying the computers, but killing him will stop the demo from ending).
"Uplink complete" appears on-screen, followed by the statement that "further data [is] required". An Internet page appears, allowing the player to order the full game.
Some of this page's text was taken from Half-Life: Uplink's Wikipedia page, before it was "merged" into a single sentence mention on the Half-Life page, and is licensed CC-SA 3.0. This site is not affiliated with Valve Software in any way.