MiniBasic is a small but usable Basic interpreter.
It is written in pure ANSI C, so it should be easy to get up and running in almost any environment. It has been used for PIC32 electronics projects, as an embedded scripting language, and as a teaching aid.
There's an associated book, and full documentation and sample programs online (go to the project's web pages).
It's designed to be easy to modify, to use the code as the basis for your own interpreter.