* RPython is still around---PyPy is written in it! Certainly it would be more friendly toward the JIT, which does best when the types of variables are consistent (which is required by RPython). Plus, RPython can be translated directly to native code and you can get rid of the JIT entirely.
* Unladen Swallow tried to do a lot of the same stuff that PyPy is doing, but on top of LLVM. It never did much better than CPython (within an order of magnitude), but if it did they were planning to merge it into CPython (of which it was a branch). PyPy is a followup to Psyco (which only ran on 32 bit x86) by the same people, but Psyco ran under CPython.
* Cython does not interpret Python---it compiles from a language that looks a lot like Python but with C-like semantics (for example, optional strong typing, and the ability to directly call C functions), and turns it into C or C++ code that can be compiled into a Python module. As such, it targets a slightly different use case.
" http://geetduggal.wordpress.com/2010/11/25/speed-up-your-pyt.... lists an example where PyPy is in fact faster than C++ with STL. Not sure how it would compare to well-optimized C, but remember that JIT warmup time will hurt you for short lived jobs."
But note a little further down the page. In the hands of a C++ coder (or indeed Cython coder) things look very different.
* http://geetduggal.wordpress.com/2010/11/25/speed-up-your-pyt... lists an example where PyPy is in fact faster than C++ with STL. Not sure how it would compare to well-optimized C, but remember that JIT warmup time will hurt you for short lived jobs.
* Can't answer this one, sorry.
* RPython is still around---PyPy is written in it! Certainly it would be more friendly toward the JIT, which does best when the types of variables are consistent (which is required by RPython). Plus, RPython can be translated directly to native code and you can get rid of the JIT entirely.
* Unladen Swallow tried to do a lot of the same stuff that PyPy is doing, but on top of LLVM. It never did much better than CPython (within an order of magnitude), but if it did they were planning to merge it into CPython (of which it was a branch). PyPy is a followup to Psyco (which only ran on 32 bit x86) by the same people, but Psyco ran under CPython.
* Cython does not interpret Python---it compiles from a language that looks a lot like Python but with C-like semantics (for example, optional strong typing, and the ability to directly call C functions), and turns it into C or C++ code that can be compiled into a Python module. As such, it targets a slightly different use case.