As mcc says, MrC/MrCpp was a compiler included with MPW, the free command-line development environment from Apple.
MrC was really great in its time. Until after new versions ceased to be released (the last was 5.0d4), it pretty reliably beat CodeWarrior in code optimization and/or code size (whichever you choose). Just by looking at disassemblies of internal Mac OS routines, you can see some of its neater features, such as early returns from a function that aborts on a simple parameter sanity check.
MrC was an original compiler, started from scratch on a new C++ framework. There were several compilers used in the early days of PowerPC Macs, causing confusion about its lineage. Apple generally does not create derivative works. GCC cannot be improved with code from MrC because it is written in C.
The MrC development budget must have been much less than CodeWarrior's. Its rate of improvement and final feature set were truly incredible given this fact.
MrC is available as a CodeWarrior plugin compiler, which was very nice. Unfortunately, there were some ABI incompatibilities caused by differences in C++ name mangling, and differences in C++ language extensions, between Metrowerks C++ and MrCpp. Also, being a late 90's era compiler, it lacks support for template features such as partial instantiation.