A very small (80k for 0.9.14) and super-fast (10 times faster than gcc) C compiler. Speed is further increased by using integrated preprocessor, assembler and linker. It is quite ANSI C compliant, and supports many ISO C99 and gcc extensions, so it is usable for almost any C project.
Of course, TinyCC has downsides. One is portability. As of 0.9.14, it seems to only support x86/linux ELF systems. Another concern is code quality. The generated code is a little better than unoptimized gcc code, but still significantly worse than optimized gcc code, and code size is a bit large. However, these issues should not stop you if you use it for development.
TinyCC is free software. It can be downloaded from www.tinycc.org.