rxvt-unicode with its Perl API [1] and rather amazing list of Perl extensions [2] on Linux OSes, puttycyg on Cygwin. (I recommend puttycyg if you've been using rxvt on Cygwin. Being able to type Unicode characters is an amazingly exhilarating experience, especially if you're lonely.)
[1]: http://pod.tst.eu/http://cvs.schmorp.de/rxvt-unicode/src/urx...