DLC and MODS
Neverwinter Nights: Hordes of the Underdark. (DRM killed it. no longer playable)
Legend of Grimrock. ² (Excellent Scripting.)
Engines (Nearly every modern engine is well represented.)
Irrlicht3D ¹ (FOSS, Source, API and documentation are super.)
Unreal ² Excellent Editor and Scripting.
Unity ² Excellent Editor and Scripting. Affordable
Minetest ¹ (FOSS, Irrlicht3D -> LUA script API)
Minecraft ¹ (Source code available for early releases.)