unluac
A decompiler for Lua 5.x
...A JAR package is available in the downloads section so you don't have to compile it. It runs from the command line and accepts a single argument: the file name of a Lua chunk. The decompiled code is printed to the standard output.
Here is an example usage of unluac:
java -jar unluac.jar myfile.lua > myfile_decompiled.lua
The current version also has pretty good support for Lua 5.0 and Lua 5.1. Support for later versions is also good if the code doesn't use too many gotos.