C3 programming language. Full C ABI compatibility, Module system, Generic modules, Design by contract, Zero overhead errors, Semantic macro system, First-class SIMD vector types, Struct subtyping, Safe array access using slices, Easy to use inline assembly, Cross-platform standard library which includes dynamic containers and strings, LLVM backend.
C3 Tutorial; has online playground.
C3 Blog by the creator, Christoffer Lernö
Also look at dlang's betterC - Better C - D Programming Language