There is a very good Lua language plugin for IntelliJ IDEA. On the RoM forums Noguai has published guides about how to set this IDE up: English VersionGerman Version
Free interface development environment (IDE) for Lua programming. Ships with the software development kit (SDK) and stand-alone compiler and linker. An addon developer must-have: Lua for Windows
AddOn Studio for WoW Has a WYSIWYG XML UI designer; just ignore the part about needing WoW