Operating Systems and Programming Languages Implemented in C
C Interpreter By Herbert Schildt
Listed in the Dr Dobbs website this is from Herbert Schildt's book Born to Code in C. It's primitive (ints only) but is still a valid cut down C interpreter.
Inferno - Operating System for building cross-platform distributions
Inferno is an dual licence (Open/Commercial) operating system for creating and supporting distributed services. Based on the experience from Plan 9 from Bell Labs, plus further research of Bell Labs into operating systems, languages, on-the-fly compilers, graphics, security, networking and portability.
Pike - A Free Interpreted C/C# like Programming Language
Pike is a dynamic programming language with a syntax similar to C# and C. It's simple to learn, is interpreted and doesn't need long compilation passes and has powerful built-in data types allowing simple and really fast data manipulation.
Developed at Bell Labs, in the mid 1980s, Plan 9 was intended to be the successor to Unix. It is useful as a grid computing platform and development still continues with an active community.
NOte the cmd folder has sources for utils.
Reactos - a Free/Open Source Windows Compatible OS
ReactOS® is a free/operating system based on the design of Windows® XP/2003. Written completely from scratch, it aims to follow the Microsoft Windows® architecture from the hardware level right through to the application level. The main goal of the ReactOS project is to provide an operating system which is binary compatible with Windows.
UBasic - A very tiny Basic Interpreter
The compressed source code for this minute Basic interpreter is only 4.5KB in size! It interprets a simple dialect of Basic (single char variables) but is an excellent example of a simple interpreter in C.