Something that really bothers me about the modern game industry is how many developers gloss over mod support and other goodies. Marketers these days are obsessed with user-generated content like live-streams, but refuse to push beyond that.
It looks like games are rapidly becoming 'Black Boxes'- So lets post about games that aren't locking down player creativity: Hostable Servers, Map Editors, Total Conversions, et cetera.
>Quake
Trenchbroom is an amazing level editor: Anyone can make Quake maps with this tool.
>S.T.A.L.K.E.R.
There are so many mods for this thing, it makes my head spin.
>Morrowind
The Construction Set is crude by modern standards, but shipped with the game and still has an active userbase.