Another vote for Jeff Buckly - great set of lungs on him that guy!
Although I am not all that keen on his music, there is no denying that David McAlmont has a cracking voice too.
Looking at the list above, it is obviously hard to separate "technical ability" from a voice that affects and moves you regardless of how polished it is (See Johnny Cash etc......) and indeed from the actual song/tune itself.
With that in mind I would like to add Bonnie Prince Billy aka Will Oldham - technically a pretty "poor" voice but it carries a warmth and honesty that I have found incredibly affecting across his music.
Mark Lanegan also has a great unique voice - all the gravel and 60-a-day lungs of Tom Waits, but with a softer crooning edge when he needs it - heard his voice described by some music press recently as that of a "Mournful Ogre"
My final name is Layne Staley From Alice in Chains - a really fantastic and unique sounding voice, not always apparent on their early or heavier work, but on Jar of Flies/Sap and especially on "Unplugged" he really shines through as an excellent vocalist.