Now playing: The Grateful Dead, "Terrapin Station" (title track from the album of the same name)
I'm not much of a Dead fan. Most of their stuff is too mellow for me. But this 16:25-long track is a big exception for me. That length doesn't come from the Dead's usual endless meandering improvisations. This is actual, genuine, bona fide progressive rock. Still mellow, but quite interesting. And those nonstop repetitions of the main theme which close the song become hypnotic in a way that few tunes can.
Another song with a hypnotically repeating end section:
The Beatles, "I Want You (She's So Heavy)"