Currently the most common implementation of Python is known as CPython, and it's the version of Python you get at python.org, probably 99.9% of Python developers are using it. However, I think over the next couple of years we're going to see a move away from this towards PyPy, Python written in Python. This is going to happen because PyPy offers better speed, more flexibility, and is a better platform for Python's growth, and the most important thing is you can make this transition happen.
The first thing to consider: speed. PyPy is a lot faster than CPython for a lot of tasks, and they've got the benchmarks to prove it. There's room for improvement, but it's clear that for a lot of benchmarks PyPy screams, and it's not just number crunching (although PyPy is good at that too). Although Python performance might not be a bottleneck for a lot of us (especially us web developers who like to push performance down the stack to our database), would you say no to having your code run 2x faster?
The next factor is the flexibility. By writing their interpreter in RPython PyPy can automatically generate C code (like CPython), but also JVM and .NET versions of the interpreter. Instead of writing entirely separate Jython and IronPython implementations of Python, just automatically generate them from one shared codebase. PyPy can also have its binary generated with a stackless option, just like stackless Python, again no separate implementations to maintain. Lastly, PyPy's JIT is almost totally separate from the interpreter, this means changes to the language itself can be made without needing to update the JIT, contrast this with many JITs that need to statically define fast-paths for various operations......
Alex Gaynor -- PyPy is the Future of Python