Fexl is the simplest and most powerful language I've been able to devise. You can do low-level system programming in it, but you can also create secure "sandboxes" to allow restricted high-level scripting. The interpreter is tiny (about 27K at the moment), and it is easy to call external C routines. The interpreter itself is written in standard C and compiled with strictness and optimization.
(Fexl is pronounced Fex'-uhl, similar to "pixel".)
Browse or install the Fexl source code. (Updated Tue 2013-11-05)