For me, YouTube has been indispensable for viewing videos of artists/songs that I haven't otherwise ever seen. A couple that come to mind:
The Byrds doing "Hey, Mr Tambourine Man"....with a special guest, Bob Dylan. David Crosby was still in the group, so what, about 1967?
Joan Baez doing an acoustic version of "Love is Just a Four Letter Word" from her kitchen in Woodside, California. This was pretty recent, but it sounds better as an all-acoustic song than it does in the Nashville gussied up version done back in 68 or so on the album of Dylan covers.
We Five doing "You Were On My Mind" when it first came out. The Honeycombs doing "Have I the Right" with the drumming of the great Honey Lantree. Jackson Browne, Linda Ronstadt and the Eagles doing "Take It Easy". Paul McCartney doing the first live performance of "A Day In the Life". Jake Shimabukuro doing "While My Guitar (ukulele?) Gently Weeps". Buffalo Springfield doing "Mr. Soul". There is so MUCH. Gordon Lightfoot in his prime. Sometimes I find myself browsing from one artist/song to the next, and I find an hour has passed.