Mortified is a group in various cities that allows people to "share their own adolescent journals, letters, poems, lyrics, home movies, stories and more." It's embarrassing, to be sure, but it's frequently also hilarious (NSFW). Recently they've set up a page to share videos of live performances, and the latest is my favorite so far. "500 Miles To Hollywood" features Elijah Wood, James Denton (Desperate Housewives), Busy Phillips (Freaks & Geeks), Kevin McDonald (Kids in the Hall) and Curtis Armstrong (Revenge of the Nerds) "helping Jason Smith fulfill his dream and bring a 2-decade-old screenplay to life."
The recent post about Joshua Bell was merely the tip of the Unannounced Performance iceberg, a phenomenon I've often marveled at. The Beatles famously did it in 1969 on a roof. Mos Def got arrested for it last year. REM reformed in 2005 at a wedding, something the Police did at Sting's 1992 nuptials. Sometime after midnight in Union Square NYC on Nov. 5, 2005, Arcade Fire blew a few lucky fans' minds. Bruce Springsteen jammed with a street musician in 1988. In 2000, Weezer took to the stage under the name Goat Punishment and U2 used to sneak onstage disguised as The Dalton Brothers. In 2005 it was rumored they'd played a Beatlesque rooftop gig in NYC, but you can't believe everything you hear. I could go on all night with tales of secret gigs and surprise busking sessions, but I'm sure you've got plenty of rare musical moments to share in the comments.