Comeau C/C++
Comeau C/C++ is a C and C++ compiler released by Comeau Computing.
As opposed to most other compilers and similarly to Cfront, Comeau C++ does not include any target machine code generator, but produces an intermediate C code and relies on an already installed C compiler for target code generation[1]. Because of this it can be used with exotic systems, for which no native C++ compilers are available.
Comeau C/C++ is built upon the Edison Design Group C++ frontend, also utilized in the Intel C++ Compiler, and owing to it, it is considered to have excellent support of the C and C++ standard. It's also one of the few C++ compilers to fully support the export keyword for exported templates[2].
Comeau C/C++ is not freeware, but it's possible to test compilation of code snippets online.