CParser: GCC-compatible C frontend with full C99 support
X10i: X10 native code compiler (for invasic architectures)
bytecode2firm: Java bytecode frontend
Fluffy: Frontend for the fluffy language
brainfuck: Brainfuck frontend (and a good tutorial)
FirmForth: A just-in-time compiling forth interpreter (also small and comprehensible)
firm-testsuite: C compiler and Firm optimization testsuite