Once an option
or an unofficial add-on
made by computer games geeks
and now something that every games developer tries to add to their creations.
Mission editors, along with map editors are one of the coolest features of modern videogames. They allow you to expand the life of your game and to put to test your creativity and game design skills.
Famous videogames for its mission/map editors are: Falcon 3.0, Warcraft II, Doom, Quake, the whole Sim games, in which the mission editor is the game ( SimCity, SimEarth), etc.
Nowadays the concept of custom videogames goes still further: avatars can be modeled and modified, skins can be added and your own sounds and graphics can be exported to simulations, first person shooters and massive online role-playing games.
Apart from enhancing identity through personalization while playing videogames, modifiable videogames have opened the doors of professionalism to many computer games enthusiasts.