Slightly of topic, but can anyone with better knowledge then myself of ruby implementations and internals explain why it hasn't caught much of a following (what it seems anyway, I may be wrong here).
To me it seems like a pretty good idea to implement Ruby mostly in Ruby, since it makes it easier to get contributions (especially to less performance-sensitive parts of the language, e.g. parts of the standard lib).
https://github.com/ruby/ruby/tree/trunk/lib
Rubinius' one (called RubySL) also includes some rewrites of standard libraries, that were originally written in C. And as you may know, Rubinius itself is just Ruby code to some extent.
mruby has some Ruby code, too. Even some parts of the core language are written in it.
Also, people that do want to work at the core of a language usually have knowledge of multiple and don't mind using C or any other implementation language.
Ruby stdlib is mostly written in Ruby, btw.
Still, with Rubinius 3.0 coming up, I'm very interested what is coming there.