[compay]

Adobe Lightroom is about 40% written in Lua (the stat comes from Wikipedia). IIRC C and C++ are used for image processing and compute-intensive stuff, but the rest of it is all Lua. I believe that would be the most widely used app written mostly in Lua, though Apache, Redis, Nginx, Vim and some other big projects support using it for scripting.

[jzwinck]

Also note that Lightroom was written before LuaJIT was really a thing; had LuaJIT come first there might have been even more Lua in Lightroom. Like World of Warcraft, Lua also provides a plugin system in Lightroom (but very few people use it or even know it exists).