Having just watched a repeat episode of Dexter featuring Keith Carradine as Special Agent Lundy, I was pretty shocked to see his name come up again in one of the iTunes Essential genres collections for the song I’m Easy, which I even vaguely remember hearing. so it must have been a fairly big hit, and I’m just now realizing what a run-on sentence this is.
I’m sure this thread idea has been done before, but it’s one in the morning and I got nuthin else.
She’s Like The Wind-Patrick Swayze
I hope these count
Beverly D’Angelo as Lurleen Lumpkin
Your wife don’t understand you, but I do
Bagged me a Homer
Bunk with me tonight
Stand by your manager
Cryin’ in my cradle
Daddy’s back
Entire threads could be dedicated to the recording careers of William Shatner and Leonard Nimoy.
Edit: Should also mention Don Johnson, whose song “Heartbeat” hit #5 on the singles chart, at the height of his “Miami Vice” fame. I remember a show on VH1 some years ago, in which they had some celebs watching videos of really bad songs, and ultimately selecting “Heartbeat” as the one video which VH1 would never show again. They even destroyed the videotape cartridge of it. 
The song “I’m Easy” was not famous because it was a big hit. In fact, it was only a minor hit. It’s famous because it won the Academy Award for Best Song. It was sung by Carradine in the movie Nashville:
The Blues Brothers’ entire catalogue. I love it and they were backed by a terrific band, but Belushi and Aykroyd were not singers by any stretch of the imagination.
Pre-Grease, John Travolta had a novelty hit called “Let Her In.” He actually had singing chops (He was in the touring company of Grease before he was a sweathog), but he didn’t use them in this treacly David Cassidy wannabe tune.
“Respect Yourself” by Bruce Willis
“Party All the Time” by Eddie Murphy
“MacArthur Park” by Richard Harris
“Wandering Star” and “Barroom Buddies” featuring Clint Eastwood
Shelly Fabres always said she didn’t have much to do with her #1 hit, Johnny Angel. She said, she couldn’t really sing, so they backed it with session singers and arangements to make her sound decent.
Her TV brother Paul Petersen had a hit with My Dad, which went to #6
Charles Manson has several songs available.
The pick of them is called “Look at Your Game, Girl.”
“Gallant Men” by Senator Everett McKinley Dirksen. Hit the top 40 and won a Grammy.
“Deck of Cards” by Wink Martindale (yes, the game-show host) was a top ten hit back in the day.
“Thunder Road”, by Robert Mitchum.
“Let The Eagle Soar”, by John Ashcroft. Sorry, but it had to be said.
Wow, you folks are digging deep! I am impressed, though I’m sure most of these songs would as well stay buried.
Don’t Give Up on Us by David Soul (Hutch in Starsky and Hutch).
Edit: Looking at Wikipedia I guess he started as a singer. I didn’t know that
“Wandering Star”, with Lee Marvin. Dude couldn’t carry a tune in a bucket.
The Unknown Stuntman–Lee Majors
Nobody’s mentioned Vicki Lawrence? The Night the Lights Went Out in Georgia?
Shocking.
Why would me mentioning that be shocking? 
Anyway, that reminded me of Kelsey Grammer singing the theme to his own show.
Benny Hill’s Ernie (The fastest Milkman in the West) topped the UK pop charts twice.
Neil Morrissey had two UK #1s with **Bob the Builder **and Mambo #5 (or Mambo 6 & 7/8ths if you prefer)
The Hoff. I guess he’s considered on in Germany, but…
NSF your sanity.
Eddie Murphy’s Party All The Time