LOL @ SHEEP VOICES!!!!!! I regularly impersonate Stevie Nicks as a goat...
Anyhow...I noticed a few that I haven't seen yet, which for me are pretty unmistakable...Note that I don't necessarily LIKE all of these, but I'll admit their distinctiveness.
PHIL COLLINS. It's the pronunciation.
ADAM DURITZ. It's the WHINE.
CELINE DION/BARBRA STEISAND.
SIMON leBON. It's this little shriekything he does at the beginning of words.
JOHNNY CASH. Low, low, low...
TOM JONES. Completely and unrepentantly cheesy.
MICHAEL JACKSON. He has a sound.
RON SEXSMITH. The tremolo.
LYLE LOVETT. The sexiest rasp in music!
ROD STEWART. When rasp goes to extremes.
BOY GEORGE. A touch off key.
I agree with many, many I've seen here, including Macy Gray, Michael Stipe, Elvis...lots and lots. (Conversely, though, maybe we should start a thread about artists you've mistaken for other artists. I've almost NEVER been able to guess Beth Orton/Indigo Girls correctly! And I've been known to mistake Travis and Wilco, Sheryl Crow and Lisa Loeb.)
Don Henley...I LOVED the duet he did with Sheryl Crow on her album...it was cheesy and poppy, but he was FLAWLESS and PERFECT! The way my husband put it: "Don Henley is about the only guy in music who can do a power ballad without sounding like a p***y..."