Lua

Lua is a computer programming language described as a lightweight language intended for writing extensions, although it can be used on its own. Its manual is only about 40 pages long.

It is freely useable, but it is copyrighted.

One of the interesting features of the language is that it is both a language and an API for accessing the runtime state of the language's interpreter from a host C program. This means that most Lua programs run as part of a host application which is usually written in C or C++. The API allows different parts of a program to be written in C and Lua and communicate values, functions, and execution between them. A function in Lua can call a function in C and vice versa.

See http://www.lua.org/ for more information.