AddOn Studio for World of Warcraft (not actively supported) - A Visual Studio-like IDE for developing WoW addons
Interface Customization Tool - Official Blizzard UI kit. Extract all the built-in interface files including source code and artwork.
WowLua - In-game Lua scripting environment. Includes an interactive interpreter, and a multi-page script editor. Available for download at WoWInterface.com.
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.