Guile library for flexible applications

Guile is a library designed to help programmers create flexible applications. Using Guile in an application allows programmers to write plug-ins, or modules (there are many names, but the concept is essentially the same) and users to use them to have an application fit their needs.
There is a long list of proven applications that employ extension languages. Successful and long-lived examples in the free software world are GNU Emacs and The GIMP.

http://www.gnu.org/software/guile/guile.html#whatisit