I haven't had the chance yet to watch all of the Hope for Haiti telethon, but I did catch Mary J. Blige perform a chill-inducing rendition of 'Hard Times Come Again No More'.
She was fantastic, and the song - written in 1854 and popularized during the civil war - had a weight and timeless message befitting the occasion. The first line is "Let us pause in life's pleasures and count its many tears, while we all sup sorrow with the poor."
This performance alone should be well worth the cost of the benefit album. You can download the album or individual performances here: http://www.cmt.com/haiti/. Most of the performances including Mary's, below, are on youtube.