Guile

Guile is the GNU Ubiquitous Intelligent Language for Extensions, the official extension language for the GNU operating system.

Guile is a library designed to help programmers create flexible applications. Using Guile in an application allows the application's functionality to be extended by users or other programmers with plug-ins, modules, or scripts. Guile provides what might be described as "practical software freedom," making it possible for users to customize an application to meet their needs without digging into the application's internals.