: No doubt, but what sort of things do they change
: from the published version? Era? That would be more
: or less cosmetic, not trivial but not the
: equivalent of a physics change. Add on
: scenarios/landscapes? Sort of like adding a track I
: guess.
The FPS world distinguishes between regular "mods", which may involve
new maps, sounds, character, weapons, gameplay mechanics, etc. but will
reuse existing elements -- and so-called "total conversions", where
virtually everything is changed. The aforementioned "most popular
online game" is Counterstrike, one of many total conversions for
Half-Life.
Half-Life is a cinematic, science-fiction action/adventure game about a
scientist escaping from a lab facility where an experiment has gone
horribly wrong (great game, BTW). Counterstrike is an online-only
military combat game with goal oriented team play featuring realistic
weapons and weapon damage. All new maps, characters, sounds, weapons
(and other items), physics, and encompassing game structure.
Another Half-Life mod called "team fortress" features entirely
different character classes with unique abilities, such as snipers,
engineers, medics, etc. There are tons of mods for Half-Life, and it's
one of many FPS with large mod scenes.