Website Utilities
WebLua - Interactive web-based Lua interpreter.
blp2png - Convert BLP files to PNG and vice versa.
Helper Applications
AddOn Studio for World of Warcraft - A Visual Studio-like IDE for developing WoW addons
Interface Customization Tool (PC Zip, Mac Zip) - Official Blizzard UI kit. Extract all the built-in interface files including source code and artwork.
Lua-WoW (PC Zip, Mac DMG) - Modified Lua distribution that includes changes to more closely match the WoW environment. See the readme files for more details.
Development Addons
WowLua (Zip) - In-game Lua scripting environment. Includes an interactive interpreter, and a multi-page script editor.
DevTools - This addon lets you examine API events as they fire, lists all the frames underneath the mouse, and provides a few other handy slash commands for developers.
EditingUI.zip - Traverse the entire frame structure, tweak appearances, export Lua or XML frame definitions, and more.