We've just released Decoda, commercial Lua IDE and debugger for Windows. We're using Lua heavily in our current game (Natural Selection 2), and we wanted a good tool to help us quickly write bug-free Lua code. We couldn't find anything that fit the bill, so we ended up creating the tool ourselves. Along the way we talked to a lot of the people who were looking for the same thing, so we decided to polish it up and turn it into a product.
Decoda has all the features you'd expect in a modern IDE (block comments, find in files, syntax highlighting, watch windows, step-in, step over, etc.), integrates with most source control systems and requires no code changes to debug Lua in your application. It works with Lua 4.0 and later as well as modified versions of Lua.