At Willie Nelson's recent Luck Reunion concert, he welcome a very special guest to perform "Rainbow Connection" with him. Willie played guitar and Kermit picked the banjo. Video below.
Kermit knows this tune well, of course. In 1979, Kermit released his version—curiously credited to someone named Jim Henson—for The Muppet Movie. It hit #25 on the Billboard Hot 100 and landed an Oscar nomination for best original song.
At the Luck Reunion, Willie and Kermit also played "Will the Circle Be Unbroken," "I'll Fly Away" and "Hard to Be Humble."