ZeroBrane Studio is a free open source IDE for the Lua programming language first released on 2012-01-18. The IDE has most features expected from a modern IDE including simple access to the compiler, color syntaxing, auto-complete, code folding, auto-indenting, debugging, and various others. As far as I know, it's the best Lua-dedicated IDE.
While looking for a simple 2D video game programming environment, I discovered LÖVE, but, after learning that it used the Lua programming language, I ignored it since I had little interest in learning a whole new language. However, after discovering that video game emulators often use Lua for scripting, I played around with the language and found it pretty easy to pickup. So, I went back to LÖVE and discovered that everyone recommended ZeroBrane Studio for an IDE since it had a good plugin for LÖVE. I used the IDE for quite awhile as I worked on my game, and found it to be both easy and powerful.