Click on the header to list all pages discussing alternative extension languages. Emacs and XEmacs are the most popular Lisp engines. As new interpreted languages develop, however, EmacsLisp need not remain the only choice. See ExtensionLanguageAdvocacy for some ranting.
For an interesting view about extension languages, take a view of Comparing Different Strategies for Adding Scripting to an Application, especially the last comment.
Not Emacs but what can we say:
See also EmacsImplementations.
Perhaps a section on AddingExtensionLanguages? would be good?