- TinyCC, SCC (Simple C Compiler) and Kefir are all fairly serious projects in active development.
- QBE is a new optimizing backend much simpler than LLVM; cproc and cparser are two of the C compilers that target it, in addition to its own minic.
- There's the bootstrapping work of stage0, M2-Planet, and mescc.
- chibicc is an educational C compiler based on the author's earlier 8cc compiler. The author is writing a book about it.
- lacc is another simple compiler that works well, although development appears to have stalled.
I think a lot of these projects are inspired by the problem that GCC and Clang/LLVM are now gigantic C++ projects. A pure C system like OpenBSD or Linux/musl/BusyBox ought to be able to compile itself without relying on C++. We really need a production quality open source C compiler that is actually written in C. I'm hopeful one of these compilers can achieve that someday.