[hercynium]

This depends on your definition of "underrated" but I'd definitely throw Perl 5 into the ring here.

Edit: Specifically, Perl 5 + CPAN is what makes it so much better than many people think. The language itself is insanely flexible, which lends itself to extensions that greatly increase its expressiveness. IMO, if you start with Moose (from the CPAN) as part of your "core library" Perl 5 becomes a very powerful tool for writing very nice code, at any scale.