Summary 8

As studying in lecture, there are many game that can be 'mod'ified by opening source in these days. Mod suggest many things, but my choice is Blizzard Entertainment.
Blizzard Entertainment develop many game that support mod for a long time. For example, Diablo (1996), Starcraft (1998). Not only allow they editing mpq files including game data, but also support editor.

Hellfire, Diablo's expansion, is developed by Sierra in particular, and hellfire don't support Battlenet. So, Hellfire is nearly mod, not expansion.
Starcraft's mod that is called Stardraft include Gundamcraft. Having highly a degree of completeness, that set the fashion.

Now, let's lay an eye on Diablo II. Diablo II's mod is clearly one of the best from the viewpoint of quantity and a degree of development. There are sites of special-handling mod and mod-development companys. ─ In interesting, by donation─

For another example, It's Elderscrolls. Elderscrolls' title menu has mod item. In other words, that support mod as add-on type. Mods are essential things, not additional things in this game.