Command-T is a pretty popular new plugin for selecting files in and below your current directory; it makes heavy use of the Ruby interface.

http://github.com/wincent/Command-T

It also uses a small C extension to Ruby for speed, because for this particular scenario Ruby is a bit too slow to be comfortable.

Since Lua is so lightweight and fast, I think it should be an alternative worth considering in situations like this one, since it offers a reasonable compromise between the speed of C and the comfort of a high-level language like Ruby or Python.