Using Lua as a configuration language is often the first step toward a fully scriptable application. Lua is licensed under the MIT license which explicitly permits commercial use. Some examples:
Neovim
Neovim uses Lua extensively
(via LuaJIT).
It can for instance load its configuration from an init.lua instead of the
traditional init.vim. Integration with Lua has sparked a
huge number of plugins
for Neovim.
MPV and VLC
Both MPV and VLC use Lua as a main scripting language.
Nginx
ngx_http_lua_module allows to embed lua into NGINX HTTP servers. It is part of OpenResty - high performance web platform based on Nginx and LuaJIT.
Redis
Redis allows adding functionality with Lua
And more
There is even browser and code editor written in Lua. For more look at awesome-lua repo.