PyPy is python implemented in RPython, which is technically a python subset. It's so restricted it might as well be a different language though.

The fact that it's written in python is often brought up in order to explain its name. But really, it's much less interesting than the fact that it has a tracing JIT. If it were called PyJIT I'd bet it would be clearer and more obvious that it's fast. And people would prob get less hung up on the distinction between python/rpython.