1 Alex Skolnick - a thrasher who plays jazz
Alexander Nathan Skolnick is an American metal and jazz guitarist who is the lead guitarist of the Bay Area thrash metal band Testament, and heads the jazz band Alex Skolnick Trio.
Alex Skolnick of thrash metal band Testament plays jazz in Alex Skolnick Trio. He's a thrasher but he's also a jazzer! - Metal_Treasure
2 Jeff Walker - recorded a country album
Jeffrey Walker is a bass guitarist and vocalist from Windle, St Helens, UK. He is best known for his work with death metal band Carcass for which he is also the main lyricist.
Bassist for death metal / grindcore band Carcass. - Metal_Treasure
3 Frederik Ehmke - a drummer who plays bagpipe and flute
Frederik Ehmke is a German metal drummer, who has been the drummer for the German metal band Blind Guardian since 2005.
He's the Blind Guardian drummer but he plays bagpipe and flute on some Blind Guardian songs. - Metal_Treasure
4 Jeff Waters - a thrasher who is reported to have ghost-written many successful country/pop ballads for other artists
Jeff Waters (born February 13, 1966) is a Canadian metal musician and founder of the metal band Annihilator. He's best known as the guitarist for Annihilator, appreciated for his fast riffs and solos. Waters is also the primary songwriter in the band. Very often he has taken on the role of lead singer, ...read more.
Source: his wiki page, ch. Legacy. I personally also heard of such rumors, and even heard of some names of country/pop artists. - Metal_Treasure
5 Jarkko Ahola - a metal singer who sings classical and jazz, and recorded an album with a Big Band
Jarkko Kalevi Ahola is a Finnish performing artist, composer and singer, best known as a metal singer. He is the vocalist, bass player and one of the three songwriters of the metal band Teräsbetoni. He also started a solo career as Ahola, a metal band.
6 Michael Poulsen was a death metal growler
Michael Schøn Poulsen is a Danish vocalist and guitarist. He is currently the singer and main songwriter of the band Volbeat.
7 Jon Bon Jovi - jazz
John Francis Bongiovi, Jr., known as Jon Bon Jovi, is an American singer-songwriter, record producer, philanthropist, and actor, best known as the founder and frontman of rock band Bon Jovi, which was formed in 1983. He is famous for his band's hit song “Livin’ On A Prayer“ released in 1986. He ...read more.
I like his cover of 'How Long Has This Been Going On' by George Gershwin. - Metal_Treasure
8 Karl Sanders - a metal guitarist and death growler who plays baglama/ saz and bouzouki
Karl Sanders is an American musician, most widely known as the founding member of the American ancient Egyptian-themed technical death metal band Nile.
He playes them on Nile albums. - Metal_Treasure
9 Christina Aguilera - jazz
Christina María Aguilera is an American singer, songwriter, and record producer, best known for songs such as "Beautiful", "Genie In A Bottle", "Fighter", "Hurt", "Dirrty", "Your Body", "Lady Marmalade", "Reflection", "I Turn To You", "Come On Over Baby (All I Want Is You)" "What A Girl Wants" and ...read more.
10 Tobias Sammet - a metal singer and bassist who can play organ
Tobias Sammet is a German metal musician, best known as the vocalist and primary songwriter of the power metal band Edguy, as well as the founder, main singer and bassist of the metal opera Avantasia.
Also, he sang jazz in the middle of a power metal song: Aina - Flight Of Torek (3.25-4.25). I think he's pretty good. Never expected this from a metal singer. - Metal_Treasure
11 Nina Hagen - best known as a punk singer, also recorded classical, avantgarde, jazz, Indian music, gospel and synth pop.
Catharina Hagen, better known by her stage name Nina Hagen, is a German singer-songwriter and actress born on March 11, 1955 in former East Berlin. ...read more.
And propably even more, she's been around since the '70s. - Martin_Canine
12 Cher - jazz
Cher is an American singer and actress. Described as embodying female autonomy in a male-dominated industry, she is known for her distinctive contralto singing voice and for having worked in numerous areas of entertainment, as well as adopting a variety of styles and appearances during her five-decade-long ...read more.
13 Reginald "Fieldy" Arvizu - a metal bassist who recorded a rap album
14 Lady GaGa - jazz
15 No Angels - jazz
No Angels were an all-female pop band from Germany formed in 2000. The group consisted of band members Nadja Benaissa, Lucy Diakovska, Sandy Mölling, Vanessa Petruo, and Jessica Wahls.
16 Jennifer Haben - symphonic metal singer who sang in a teen pop band
Ah...this partly explains why she sounds like a pop singer in that symphonic metal band (Beyond The Black) - Metal_Treasure
17 Björk - sang in an alternative rock band
Björk Guðmundsdóttir, known mononymously as Björk, is an Icelandic singer and songwriter. She is most famous for her eclectic music styles and experimentation on electronic pop music.
18 Marius "Marley" Ley - producer of experimental hip hop/electronica group Dat Adam, recorded a solo EP that contains him solely playing the piano.
19 Wacław "Vogg" Kiełtyka - accordion
Guitarist of technical death metal band Decapitated plays accordion. Moreover, he attended the Music Academy of Poland as an accordionist. - Metal_Treasure
20 Sebastian Lanser - a metal drummer (Obscura) who played keyboards for Belphegor
21 Sebastian Lanser - a metal drummer (Obscura) who also plays jazz and jazz-metal fusion