Some great voices mentioned here already.
I grew up a huge Pink Floyd, Rainbow, Purple, Uriah Heep, Nazareth, Nugent, Bad Co. fan.
Love the voices that came/went through those bands.
From more recent times, Chris Cornell from Audioslave and Weiland are also easy to listen to.
The dude from Disturbed has an amazing voice.
A couple of personal favorites of mine are: John Doe from X (some might consider that more punk oriented, perhaps), Craig Bloxom (from Australia's Spy V. Spy) and my all-time favorite band (Clutch): Neil Fallon
Neil Fallon just belts it out and uses no backup vocalists. Live - the guy just kills.
Clutch's version of Mississippi Fred McDowell's "Worried Life Blues":
http://www.youtube.com/watch?v=kx6FV2qR2TY&feature=channelSteve