Judee Sill's 'last' album, "Dreams Come True" was all demos, of varying degrees of quality.
They put out a Johnny Smith album that he recorded in his living room on solo acoustic guitar, that he never meant to be released. It was a double CD set called "Legends" released on Concord Jazz.
One side contained Smith playing Spanish Classical pieces with a pick(!), and the other side features George Van Eps, also playing solo guitar.
JS's idea of demos is our idea of perfection.
This might not be “cool”, but I loved the Dave Matthews Band’s “Lillywhite Sessions”. I believe were leaked pre production recordings of an album they decided to scrap (but then eventually re recorded and released).
It really reminds me of my childhood and discovering music. I prefer the darker tone of the original lyrics before some were changed for the officially released album. And the songs that didn’t make the official album are awesome to me, not sure why they were cut!