Alas. No one's mentioned SNOBOL or SPITBOL. I'm sure modern coders would consider them esoteric, but for me they were kind of delightful back in the day.
I would not consider SPITBOL esoteric since it was not intended as such — but the original INTERCAL-72 interpreter was written in SPITBOL, so the language is quite important to esolang history (and deeply weird, besides).
