Billy Holiday and Sarah Vaughan are two greats from the jazz era. Nina Simone, Ray Charles and Frank Sinatra, are some of my favorite singers from the 50s and 60s. Bob Marley, Muddy Waters (yes, Muddy Waters could sing!), and Aretha Franklin are some of my favorites from the late 60s and 70s...
As for today's singers, I really enjoy Lauryn Hill as a singer. I hope she will continue to make music, but what she has produced, vocally, since 1998/1999 has been really impressive. I've also liked what I've heard from Bruno Mars (I don't know if that's corny or not. I'm out of the "cool loop" for sure. All I know is he brings a strong singing voice to pop music.)
I also like Brandi Carlisle's singing, though in her case I think her most impressive talent is song-writing. She can definitely sing though.
Patti Griffin is another impressive contemporary singer. If you like classical singers, Kathleen Battle is someone to look into.