What do the following artists have in common? Bruce Springsteen, Joan Baez, Dave Matthews, Arlo Guthrie, Emmylou Harris, Kris Kristofferson, Roger McGuinn, Taj Mahal, Steve Earle, Ani DiFranco, Tom Paxton, John Hall, Kate and Anna McGarrigle, and John Mellencamp.
Why, they all turned up for a benefit concert to celebrate Pete Seeger's 90th birthday Sunday night.
Here's Mellencamp singing "If I Had a Hammer."