As far as tradtional desktop apps go, over 40% of Adobe's Lightroom was written in Lua. If you read the front matter in the link then you'll see their forward to the book.
Wikipedia has a list of Lua apps as well. It seems fairly incomplete... if I recall, Lua has gone into space on at least one commercial satellite :)
Lua is the scripting language for nmap, wireshark, various embedded processors such as lego mindstorms, and dozens of high-profile games.
A group of us are working on a Jabber/XMPP server in Lua.
Although popular belief seems to be that Lua is really only suitable for embedding in applications, I think it really deserves the chance to be more than that.
Our server project is hopefully on track to showing that this can be done, because Lua is a perfectly capable language with a very advanced implementation. It deserves a second look from anyone interested in picking up a flexible new language to help make them more productive.
Lua is used in many games for scripting, including one that I worked on. I think the "killer feature" for games is coroutines which allow many scripts to run in parallel and across frames, as wait functions can save the stack and later resume execution. Simple enough for non-programmers (level designers) to understand and build the occasional script.
