Postal is a computer game made by Running With Scissors and published by Ripcord Games in 1997. In 2003 there was a sequel, Postal 2. Director Uwe Boll has bought the movie rights for the series, and has produced a film of the same name.
Postal is a 3D shooter with mainly isometric, but also some top-down levels featuring hand-painted backgrounds. Gameplay and interface are similar to first-person shooters of the time in most, but not on all counts:
Movement is always relative to the orientation of the player character ("Postal Dude"). The player therefore must always be aware of the direction the character is facing, which can be difficult on the isometric maps. There are eight weapon slots, each with a fixed amount of maximum ammo. The default weapon is a weak machine gun with unlimited ammo.
Contrary to first-person shooters, however, the goal is not just to stay alive and reach the next level, but to kill a given percentage of the armed NPCs on the map. Only then the exit to the next level is activated.
There is no plot as such. The presence of a moving van on the first level suggests that the Postal Dude has been evicted from his home and is therefore "going postal", but no background story evolves during the game.
An add-on to the game called Special Delivery was made in 1998 which featured four new levels (department store, shanty town, earthquake zone and vacation resort), new characters, new sounds, new modes of play (co-op and a refined deathmatch). Original and add-on were released together as Postal Plus in 2001 and ported to Linux by Loki Software.
Source: Wikipedia, "Postal (video game)," available under the CC-BY-SA License.