Janis Joplin and Ron "Pigpen" McKernan
One of the more amazing things available on the Internet for free is the Grateful Dead concert collection on Archive.org.
There are two types of Dead concert recordings - downloadable recordings made by fans, and streaming recordings taken directly from the soundboard.
Of the soundboard recordings there are a few made by the late, legendary Owsley "Bear" Stanley, but to me the two most interesting recordings are two appearances that Janis Joplin made singing on the Dead's "Turn On Your Lovelight" with one-time lover Ron "Pigpen" McKernan.
As Wiki says Turn On Your Lovelight was a staple of Dead live performances. Bobby Bland's original version here.
Joplin appeared with the Dead twice:
the Fillmore West performance on June 7, 1969
the Euphoria Ballroom performance on July 16, 1970.
Joplin would die three months later of a heroin overdose. McKernan died in 1973.
The 1969 performance - Joplin comes in about 4 minutes into the Lovelight recording - is more disciplined than the 1970 version although slightly longer at 20:57 versus 18:04.
According to the Wiki entry on McKernan, Joplin didn't like the Grateful Dead's "jamming" style, and you can sure tell on the 1970 recording - after an extended jam at 13:41, she cracks "it ain't music, but it's alot of fun."
In Janis's defense, you can understand how her singing style wouldn't work well with the Dead's endless jams. Joplin threw everything she had into her performances, and so from the point of view of sheer human endurance, it's better for her songs to be short and tight as possible. A quick survey of Joplins' live recordings available on Youtube indicates that seven minutes was the upper length limit of her songs.
Finally she decides to take control of the proceedings so she can give herself something of a dramatic closure, rather than just allowing the song to trail off into a jam cloud. At 16:21 she goes:
Hold it! Sustain, sustain, sustain!
The music stops. Then she does her big trademark lo-lo-lo-love finish.
Fun fact - that's 5-year-old Courtney Love sitting next to Pigpen on the back cover of the Dead's Aoxamaxoa.