A comprehensive C11 frontend for QBE by Michael Forney. cproc is known to bootstrap GCC and is used as main C compiler in the oasis Linux distribution. cproc is able to compile its own source code (bootstrap).
A systems programming language by Drew DeVault that aims for simplicity, stability, and robustness. Hare comes with a well furnished standard library. It also does its best effort to save the programmer from notorious C footguns (null pointer dereferences, integer promotions). Hare has been using QBE as backend from the beginning.
A complete set of tools for C99 development (libc, compiler, linker) from Roberto E. Vargas Caballero. The compiler uses QBE as one of its backends.
A C compiler from Benjamin Rosseaux that reuses the IL design and many passes from QBE.