Disclaimer: I am CTO of Kong.

If you want to look at a large OpenResty/Lua application from a code perspective you could take a look at Kong[1] - which is the most popular public codebase built on top of NGINX/Lua at the moment.

CloudFlare also uses the same stack but most of their codebase is proprietary.

What really makes Nginx/Lua a good combination is primarily LuaJIT[2] support, a much faster implementation of the Lua VM.

[1] - https://github.com/Kong/kong

[2] - http://luajit.org