Lua is particularly well suited to be used as an embedded scripting language. Many existing software packages leverage Lua to provide users with the ability to the base of the software. All of the software listed below can be extended using Lua.
Scriptable software
CEGUI
Crazy Eddie's GUI (CEGUI) is a user interface system written in C++, often used for video games. CEGUI offers a Lua backend scripting module, which allows user interfaces to be built using Lua. The CEGUI library is easy to embed, and the Lua API it exposes allows for Lua to issue callbacks to the application that is embedding it.
The website is http://cegui.org.uk.