You can find many of these addons at the site WoWInterface.com:
The actual base framework for World of Warcraft's interface is written in Lua, and can be extracted from the game's data files using a tool provided by Blizzard.
More information about WoW's use of Lua and the interface API can be found on WoWWiki.com:
There is a distribution of Lua available (created by cogwheel) that includes everything required to install Lua locally for addon development:
This includes wowlib, luasocket, and the standard Lua 5.1.1 Win32 binaries.
There has also been a book written about writing addons for World of Warcraft, called "Hacking World of Warcraft" by Daniel Gilbert and James Whitehead. It is available through [Amazon.com].
Also see #wowace and #wowi-lounge on irc.freenode.net. Archives are available at the following URLs: