alt.pub.coffeehouse.amethyst (This is somewhat low-traffic but is unique; people write in the third person imagining themselves stepping into a friendly coffeehouse)
Surprised to see alt.arts.poetry.comments on this list. Yes it remains active, but the regular posters there[1] are for the most part the remnants who triggered the Great Poet Flight to various online (moderated) forum venues 20 years ago.
Some of those troll fights were fun. Some even involved poetry.
Live: rec.arts.sf.written -- discussion of written science fiction and fantasy
Nearly dead, but if you post cogently, someone will respond: rec.audio.high-end -- discussion of high-quality (not necessarily high cost) audio reproduction
I follow comp.os.vms
and sci.electronics.design is always hot. I think there's probably a lot of little corners of usenet which are still pretty active. I was sad to see comp.dsp die off, but it's been equally more active by the same people on stackexchange.
