PicoC is a very small C interpreter for scripting. It was originally written as a script language for a UAV's on-board flight system. It's also very suitable for other robotic, embedded and non-embedded applications.
The core C source code is around 3500 lines of code. It's not intended to be a complete implementation of ISO C but it has all the essentials. When compiled it only takes a few k of code space and is also very sparing of data space. This means it can work well in small embedded devices. It's also a fun example of how to create a very small language implementation while still keeping the code readable.