[curun1r]

One that's not listed there but might be a good candidate for a Rust project is miri [1] since it's both interpreted and still Rust. It's still pretty rough, but the approach it takes should perform pretty well and won't require people to learn a new language.

[1] https://github.com/solson/miri

[stevedonovan]

It is outrageously fast (at least compared to compile) but would need integration via libffi to speak to the rest of the universe. This is not a priority for the maintainer, since he's exploring compile-time metaprogramming (where you want a very restricted sandbox)