S-Lang
A small but highly functional embedded interpreter. S-Lang was a stack-based postfix language resembling Forth and BC/DC with limited support for infix notation. Now it has a C-like infix syntax. Arrays, stings, integers, floating-point and autoloading are all suported. The editor JED embeds S-lang.
S-Lang is available under the GNU Library General Public License. It runs on MS-DOS, Unix, and VMS.