Many people think once an Actor makes an attempt to the music business, then results will end up being sour. Still it's not always true, since there were some actors who released some pretty successful stuff and some others even gave up their acting career to pursue a career in music instead (or vice versa).
(In alphabetical order)
1) Cassidy, David
One of the most famous teen idols of the 70s, both as an actor and as a singer. He started as an actor though, and soon he released his first album, Cherish. His most famous role was the one of Keith Partridge, in The Partridge Family series. His music was more popular in the UK than in US, and his song Daydreamer, reached #1 in the UK charts.
2) Corley, Al
Mostly known for playing Steven Carrington in Dynasty series, Al Corley made a few albums during the 80s, shifting towards a Hi-NRG direction, with Square Rooms being the most known one. The title track was a big club hit in many European countries, especially in France, and the song Cold Dresses which was released as a single as well, was a big hit in France too.
Divine was a cult icon during 70s and 80s, mostly famous as being a drag queen. Divine was actually a character who appeared in cult movies like Multiple Maniacs and Pink Flamingos. He used this persona during his singing career and he released some Hi-NRG classics during the 80s with Shoot Your Shot, and You Think You Are A Man being the most known, which were Top 10 hits in certain European countries.
4) Donovan, Jason
Started as a teen actor during the 80s mostly famous for his role Scott Robinson in the Neighbors TV series, where he was co-starring with fellow Australian singer/actress Kylie Minogue. His music career started during the late 80s, and he went to be highly successful with his debut Ten Good Reasons, which reached #1 in the UK album charts, including 3 #1 hit singles as well, Especially For You (duet with Kylie Minogue), Sealed With A Kiss, Too Many Broken Hearts.
5) Hasselhoff, David
He's been involved in many TV series and movies, but definitely his most famous role was Michael Knight from Knight Rider series. He's also known as co-writing and starring in the Baywatch series as well. But not much known are about his involvement in music. Surprisingly though, he's fairly popular around Germany, Austria and Switzerland area where his song Looking For Freedom, reached #1 in those 3 countries. But his best work is definitely the album Night Rocker, which was his debut in his music career.
6) Imbruglia, Natalie
She started as an actress in the Neighbors series as well, as Beth Brennan. Her acting career wasn't long-lived, and since mid 90s she followed a singing career with Torn being her most successful song, reaching #1 in many countries. What many people don't know is, that Torn is actually a cover version of a song by Ednaswap, an American Alternative Rock band.
7) Johnson, Don
His most famous role was James Crockett from Miami Vice, or Nash Bridges from the same titled series, What is less known though is his involvement in music. He released 2 albums during the late 80s, Heartbeat and Let It Roll. The title track from Heartbeat was a Top 10 US Hit, peaking #5, and a Top 10 in many other countries, and there was a nice power ballad included as well, Voice On A Hotline, which was only a moderate hit in Netherlands, peaking at #59.
8. Minogue, Kylie
Like Jason Donovan, she gained attention first by starring in the Neighbors series as Charlene Robinson.
A few years later, she released her debut Kylie, which was a big success and her song I Should Be So Lucky, was #1 in many countries. Since then she followed a successful career as a singer.
9) Murphy, Eddie
Famous actor and stand-up comedian as well, he's famous for participating in many movies and series, and a cast member of the Saturday Night Live as well. His most famous hit in music, is definitely Party All The Time, which reached #2 in Billboard Hot 100 charts.
10) Newton-John, Olivia
She gained attention with her Olivia role from the movie Tomorrow, and other famous roles of her were Sandy Olsson and Kira, from Grease and Xanadu respectively. Her music career was also famous where she scored several #1 hits in US Adult Contemporary charts, with songs like I Honestly Love you, Have You Never Been Mellow, Please Mr. Please.
11) Nunn, Terri (Berlin)
Although she's more popular as the singer of the Synth-Pop band Berlin, instead of an actress, she had some involvement with acting during the late 70s, with her role as Jeannie from Thank God It's Friday, being the most popular.
12) Springfield, Rick
Although he started as a musician his acting career is not unknown. His most known role was Dr. Noah Drake, in General Hospital series. Although his debut album Beginnings went well, his other 3 albums from the mid-70s (Coming Book Heroes, Mission Magic!, Wait For Night) lacked any success, so he decided to pursue an acting career instead. It was big success of his Working Class Dog album,that made him make a comeback to the music scene. But his best album would be Living In Oz, in my opinion, which's an AOR classic. He is included in this list, since there was a time when he was more than an actor than a singer.
13) Stallone, Frank
Sylvester Stalone's younger brother, yet not so popular as his older brother. He didn't have any significant role, maybe his Black Knight role in the Roller Blade Seven trilogy. His most successful song was Far From Over which reached #10 in the Billboard Hot 100 charts and it was featured in the OST of the movie Staying Alive.
14) Travolta, John
His roles Tony Manero and Daniel Zuko from Saturday Night Fever and Grease respectively were those that made John Travolta a phenomenon by the late 70s. The OST from Grease was a huge success and it reached #1 in the US album charts. You're The One That I Want and Summer Nights (both duets with Olivia Newton-John) reached #1 and #5 respectively, and he had a song where he sings solo, Greased Lightnin which was a moderate success, peaking at #47 in the US charts but it was bigger in the UK where it reached #11 in the UK charts.
15) Zadora, Pia
Mostly famous for her roles Kady Tyler in Butterfly, and Jerilee Randall in The Lonely Lady. Although she started way before (during 60s) as a child actress. Her most famous song was When The Rain Begins To Fall, which was a duet with Jermaine Jackson. It was a big club hit in Europe and reached #1 in some countries as well.