Rebash is a simple C library which embeds a ruby interpreter inside the bash shell. This powerful construct allows various parts of bash to be redefined or extended.
Currently the only supported extension is syntax highlighting for the interactive shell. To use it, build the extension:
$ make -C ext/rebash
then run rebash:
$ ./bin/rebash
Planned future features include modifying existing bash syntax, adding new shell builtins, and a new function syntax which is half-bash and half-ruby. Of course, each of these features depends on how feasible each one turns out to be.