Lua. It's has everything a good C project should have: small size, simple build system, portability by using the simplest constructs and not ifdefs, a clear and well define scope that none dares trespassing.
When I used this library, I was impressed with how their design not only kept their own code clean, but made it incredibly intuitive and fun to write clean code on top of their API. Coworkers also looked at that code years later and went out of their way to give positive reviews of Lua.
