To shamelessly rip the S-lang page off;

S-Lang is a multi-platform programmer's library designed to allow a developer to create robust multi-platform software. It provides facilities required by interactive applications such as display/screen management, keyboard input, keymaps, and so on. The most exciting feature of the library is the slang interpreter that may be easily embedded into a program to make it extensible.

And it is easy to embed! A couple of simple function calls in your C program and you will have a working S-lang interpreter. It is then quite simple to allow C functions or variables to be available to any S-lang scripts your program interprets.

Jed, Slrn, Mutt and Lynx all use S-lang.