Having read and listened my way through loads of wonderful music here, much new to me, I might offer another dozen answers. Is the Lee Michaels and Frosty album below prog rock? Chicago, Jethro Tull?Yeah, and ask ten people what Progressive Rock is and you'll get, well at least 2-3 different answers.
Genesis - Carpet Crawlers:
I'm going to interrupt the nostalgia with some more current math rock. Elephant Gym (from Taiwan).
All three are great, but the bassist is a real treat:
Oh, that’s so cool! That’s the kind of stuff I’m always looking for. Somali Yacht Club and Paint the Sky Red are two other current bands in the same vein. Here’s something from the latter, ffwd a minute or so to get past the intro.
In between the Airplane and Starship days, Paul Kantner, Grace Slick, and friends like Jerry Garcia did some very interesting side projects. This was one of them - "Sunfighter" - the song is Holding Together: