Valve's debut game, Half-Life, was a huge public success and a major influence for the first-person shooter games, even many years after it was released. Launched in November 1998, it sold over 20 million units in the next 6 years, and is considered a classic of the genre. It also was one of the first games that introduced scripted scenes and removed the loading screens (admittedly, with some tricks), an important technology advancement for that time.
The story evolves around Gordon Freeman, a theoretical physicist working in new Mexico at Black Mesa Research Facility, a scientific lab similar to the famous Area 51. The game starts with one of his experiments ending in disaster, blowing up the facility and opening an inter-dimensional rift. The rift allowed hostile aliens from a civilization named Xen to enter the facility and start killing people. Gordon had to fight aliens and the Special US Military Forces dispatched a facility to eliminate both aliens and facility personnel. Freeman manages to convince the lab personnel to help him against the aliens, and is teleported to Xen world to kill Nihilanth, the creature that maintained the rift open.
In his quest, our hero has to solve multiple puzzles, travel through all the facility, and most of the time shoot everything that moves. He is also helped by a mysterious man, who appears many times and seems to monitor his actions. Like any first-person shooter, Half-Life offers a large arsenal of weapons, from the classic Magnum revolver to the exotic tau cannon and gluon gun, both built by the facility researchers.
The multiplayer mode, which allows players to compete against each other, is another great feature of the game, very appreciated by the public.
Half-Life had two expansions packs, both made by Gearbox Software: Half-Life: Opposing Force (1999) and Half-Life: Blue Shift (2001). A sequel, Half-Life2, was launched in 2004 and also had Gordon as hero, this time in a different adventure.
More than 10 years after, Half-Life remains a reference game, still played by millions of fans and highly regarded by the critics.