Let's face it, Trent Reznor doesn't strip down often. But back in 2006, the Nine Inch Nails brainchild showed up at Neil Young's annual non-profit Bridge School Concert sporting a beard to deliver an extremely rare unplugged set. Now, NIN devotees can load those stripped down approaches to "Fragile" and "Hurt" onto their iPods with today's release of 'Bridge School Collection, Vol. 4' via iTunes.

Aside from Reznor's heartfelt offerings, the compilation also counts live tracks by My Morning Jacket, Band of Horses, Death Cab For Cutie, Regina Spector, Norah Jones and Tegan and Sara, who posted the tracklist on their website. Noting that the Bridge School directly benefits children with severe speech and physical challenges, Sara writes that she and Tegan have repeatedly appeared at the concerts over the years and that it is "an absolutely incredible experience. Both professionally and personally."

Attesting to the variety of acts Young has attracted to his Bridge School concerts over the years, 'Vol. 4' also boasts everyone from Ministry -- captured back in 1994 -- to John Mellencamp, Tom Waits & Kronos Quartet, Jerry Lee Lewis, John Mayer and, um, Josh Groban.

If Groban's appearance -- culled from 2008 -- seems a little far fetched, his stellar collaboration with the Master of Ceremonies is worth seeking out. Along with Young, they turn out one breathtaking rendition of Young's 'Harvest Moon.'

Each October, Young and his wife Pegi host the Bridge School charity concerts at the Shoreline Amphitheatre in Mountain View, Calif. Traditionally, most of the performers go acoustic for the occasion, leading to special moments as they raise money and awareness for children with special needs.