I have to agree about Alison Krauss (listening to the AK/US Live album as I do frequently, I'm always amazed by the utter perfection and vocal purity in her performance. It's one thing to sound flawless on a studio recording, quite another to pull that off in concert.) Also Eva Cassidy. Oh, and Lisa Gerrard...there's a reason her voice is on so many movie soundtracks!
Now, here's a shocker and something that may get me in trouble: one of the most powerful and intense voices I've heard in recent years is the singer P!nk. Most of what she does may not show it off, but when she belts out a high note it is absolutely incredible, and I don't for a moment attribute it to AutoTune or any such nonsense. She simply has an incredible instrument.
One last thing: I also have to agree with those who say that it's not just the voice but how you use it. There are plenty of singers (Celine Dion comes to mind) who have quite an amazing instrument, but whose vocal technique/expression leaves me cold or downright repulsed. As someone said above, there's a certain self-indulgence that really turns me off with some singers. I used to love Tori Amos, who has a fantastic voice, but her performance devolved to the point where every song was basically a long, incomprehensible, almost masturbatory vocal puddle. I couldn't relate to it any longer.
*sigh* Here I am, a full-on curmudgeon at age 36. So be it. ;)