Foo Fighter Dave Grohl has teamed up with Jack Black of Tenacious D, and actor Val Kilmer and formed the band Sweetriver and The Huckleberry Dogs. Now, the band isn't really a supergroup per-se, they are a fictional band in the Yahoo web series Ghost Ghirls.
In the show, the trio play a Southern country rock band from the seventies that died in a plane crash who return as ghosts to haunt a recording studio. The Ghost Ghirls themselves, played by Amanda Lund, and Maria Blasucci, are faced with the task of helping the band finish their last song so that they can cross over in to the after life.
In order to make the band legit, the trio has actually recorded a couple of tunes. "Georgia Peach" and "Back to the Beginning" are both available on iTunes.