Elton John.
Waddayamean he’s not an actor? He got a 2 hour interview on “Inside the Actor’s Studio.” James Lipton wouldn’t dare put a one-shot actor on such an important show.
Vanessa Williams (though, to be fair, she started as a (disgraced) beauty queen, then became a singer, then an incredible actress)
Jennifer Hudson, although she made the transition pretty quickly
David Soul
Shirley Jones
Julie Andrews
For the record, they were all singers before they went into musical roles, then into straight acting.
Jack White (of the White Stripes); Cold Mountain
Justin Timberlake has gotten good reviews in his acting gigs. The SNL crew were reportedly very impressed by what a good actor he is.
Grace Jones.
Harry Connick , Jr.
Tim McGraw (Friday Night Lights, Flicka)
Once again, you whippersnappers missed.
Elvis.
Nope.
She was a highly respected actor before she became known as a singer, unfortunately tarnishing her reputation as an actor at the same time. See *Out of Sight *before you write her off. Hopefully she’ll eventually recover some street cred. (Though it probably doesn’t help that even before that she was a dancer on In Living Color . . .)
Psst, post 14.
Christopher Walken was a song-and-dance man before becoming a mainstream actor.
thwartme
/this thread needs more cowbell!
If we’re counting rappers, then LL Cool J (now credited as James Todd Smith, I think), Ice Cube, and Ice T all have quite respectable acting careers. Ice Cube is a particularly appealing actor.
Yul Brynner worked as a nightclub musician, playing guitar before he became an actor
Tom Waits (Short Cuts, Mystery Men, Down By Law)
John Lurie (Down By Law, Stranger Than Paradise, Wild at Heart)
What? No Dean Martin? No Bing Crosby?
mandy Patinkin
I’ve been surprised to see Chris Isaac and Lyle Lovett turn up in acting roles.
Alice Cooper
He starred in the world’s worst movie. It’s called monster Dog and that about describes it. It was originally made in English, in Spain, and then the master soundtrack was scrapped and the whole thing was dubbed in Spanish. THEN, the whole thing was wiped again and the English was restored, but with different actors doing the voices. The result is, uh, unsettling. On top of that, it’s a really, REALLY, lame wherewolf movie.
Alice is also in Prince of Darkness as the leader of the zombies and one of the Nightmare On Elm Street movies as Freddy’s evil stepfather. He’s played himself too, most notably in Wayne’s World and Roadie. Oh, and he was in the atrocious Sergent Pepper’s Lonely Hearts Club Band. On TV, he’s been in Something Wilder and That 70’s Show and of course The Muppet Show, not to mention Welcome To My Nightmare which did involve some acting. Whoops, almost forgot The Snoop Sisters!
(FaithFool, beat ya to it! 
)