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.
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).
VLC, MySQL (and MySQL Workbench), Wireshark, AwesomeWM, NMAP, NGINX, Ogre, CEGUI (and most corporate game engines), OpenWRT, Recent TI calculator (bye bye TI basic)
Many more, but it is what come to my mind right now
Games can't be emphasized enough. Grim Fandango and Escape from Monkey Island in the late 90's were the first known corporate games that used Lua. After word got out at GDC, the entire industry started using Lua. Big profile hits like World of Warcraft and Angry Birds make Lua sound like something new in the video game industry, but Lua was already de-facto standard. If you buy a game today, there is a greater chance that it uses Lua than it not.
Nothing is pure lua, you could probably use GObject-introspection to write a GTK app, but nobody do it.All of these apps have _some_ lua code, but it is nowhere close to 10%, I will give you that.
While I cannot pretend it is significant, it is new/fresh/recent. I'm just going through a transition to the GMime mime library, but beyond that the code is stable and I've used the client for the past few month or so, replacing mutt as my main client.
Now that I'm using it I'm finding interesting things to do all the time, using Lua has allowed me to move from having a mostly-extensible client (mutt) to having a __very__ extensible client.
Lua is an industry standard in the video games business. The scripting language used is usually Lua or Python. Examples of Lua-scripted games: Civilization V, King's Bounty, SimCity 4, Dawn of War, Crysis etc.
A lot of WoW itself seems to be written in Lua. A bug in Lua would occasionally allow a cheats in WoW, so it got heavily tested by motivated folks. I'd call that a significant deployment.
It's more fair to say that a lot of the UI for WoW is written in Lua, and most of the API available to that UI is also available to addons that end-users write for the game.
