Andy Hurley of Fall Out Boy played in both Killtheslavemaster and Racetraitor, and Pete Wentz played in the straight edge hardcore band Birthright.

Niko "Skorpio" Sirkiä - Thergothon as well, played for This Empty Flow.

Andy also drums in Enabler

http://www.youtube.com/watch?v=qNznKxcLEOA

Andy also played in Earth Crisis for a tour or two after Fall Out Boy broke up.

Juan Aristizábal from Ekhymosis. The band itself became a pop band, but their earlier stuff was thrash in the Metallica vein. Juan eventually went on to a solo pop career. Shame because the pop stuff sounds uber-generic to me and the thrash stuff was decent second tier material.

http://www.youtube.com/watch?v=L-WZcQSV9zo as Ekhymosis

http://www.youtube.com/watch?v=lhx515lsZGU as Juanes

Johan Längquist of Candlemass is in a pop band. I tried youtubing a video but I couldn't find one. I'm sure they're out there though.

Ernie C of Body Count produced Black Sabbath's last album before the 1997 reunion with Ozzy and most of the original lineup. Not strictly speaking a performing role but it counts, Ice T did a spoken word piece on one of the tracks as well.

Wasn't Shannon Larkin the drummer for Ugly Kid Joe as well before he joined Godsmack? Not quite "more extreme", but still......

Yup, indeed he was.

Haha, forgot about him - his early solo stuff was also semi-glam rock stuff, like a super-super-super poppy version of Bon Jovi.

For a brief period of time, Paul Dianno played some AOR stuff in between Maiden and his more metal/hardcore bands. http://www.youtube.com/watch?v=oD-TSWcu2F8

Tony Choy, who played in Cynic, Atheist and Pestilence, now plays in a latin pop group called Area 305.

THANK YOU! While that part always feels really awkward in an otherwise solid song, it bugs the crap out of me when people say that Ice-T is rapping in the middle of The Illusion Of Power. Just because he's a rapper doesn't mean that it's a rap. That and it's too short and doesn't have any flow.

Deathstars used to be Swordmaster and the drummer was in Dissection.

I'm surprised no one's mentioned The Kovenant originally being Covenant.

According to Davey, "Marty Friedman left us to go play Japanese pop music!"

Also, before Dream Theater, James LaBrie sang in a band that is as far away from Progressive metal as you can get. A sentimental glam pop outfit going under the name Winter Rose.

I reckon Ghost (Sweden/Occult Metal) is quite popular now. Papa Emeritus (Lead Ghoul or frontman whatever he is) is actually a skinny dude named Tobias Forge. He's the frontman of Repugnant (Sweden/Death Metal). He has also played in a alternative/punk rock oriented group called Subvision.

The guy has the lyrical credits, it doesnt mean he is Papa Emeritus. Tobias also played in the early incarnation of the glam rock group Chrashdiet.

There are lot of speculations about that. But I'm pretty sure Tobias is Papa Emeritus.

Not sure if he has already been mentioned, but as far as I'm concerned Devin Townsend has mastered both pop music and extreme music. Strapping Young Lad's Alien and his solo album Ki demonstrate my point beautifully.

Tobias was the singer in Subdivision. Listen to their song Psycamore.

https://www.youtube.com/watch?v=NExD7pW7nUc

Now listen to Ghost's song Elizabeth.

https://www.youtube.com/watch?v=CaAb421KUhk

That is proof in the pudding that Tobias Forge is Papa Emeritus.

If you're talking about the Swedish EBM group Covenant, it's not the same guys.

Not that anybody cares about this band now, but back in the 90s and when I was in school, I remember Sugar Ray played some heavy music. I can't confirm this via googling or through wiki right now, but that's my hunch.

They used to play Nu-metal, I completely forgot this. Haha.

Floored was the shit. Sugar Ray is still good for a spin once in a while.

One of Crashdiet's ex-singers, H. Olliver Twisted, has gone on to the much glammier and poppier band Reckless Love.

The shitty pop band Soil had/has members of Oppressor and Broken Hope.

Exactly what I came here to say. Almost unbelievable the way a top-tier quality technical dm band turned into that. I've listened to quite a bit of Soil, searching for redeeming quality...but alas...

I was talking about the Norwegian black metal Covenant changing their style to electronica-influenced industrial metal with tinges of black metal kinda sorta still there at times and their name along with it to The Kovenant.

In Flames played decent melodeath but then decided to flap shit.

J.Mascis of Dinosaur Jr. (Although Dino Jr. is not quite popular in that sense) has played drums in doom metal band Upsidedown Cross (US) and still plays drums in Witch (US/Stoner, doom).

At least Shaun Glass came to his senses and reunited with Broken Hope. Now Tim King needs to take his balls out of his purse and bring back Oppressor.

Dana Strum, better known for as bassist in glam metal bands Vinnie Vincent Invasion and Slaughter (US), produced an album by speed/thrash band Détente in 1986.

The former drummer from this band went on to play this... stuff.

Finnish death metal band Amoral getting an Idol singer from that country and changing to really crappy power metal.

The drummer from Dissection plays in Deathstars.

The current vocalist for Cloven Hoof appeared on The X Factor in the UK this year. He's now apparently begun a pop career. Also, somewhat amusingly, his X Factor "career" was funded by a convict described at the time of The X Factor's airing as "The most wanted man in Britain".

Shaun Glass, long ago, used to visit the AOL Metal chats and basically crapped on his time in Broken Hope, so I'm guessing he did it to make a couple bucks doing festival dates and stuff like that more than because he wanted to play death metal again.

Liv Kristine and Anneke van Giersbergen played in metal bands (not that extreme but still metal) and then started solo-careers making a kind of pop/rock music.

Liv once said Madonna and Black Sabbath were her biggest influences actually

Yep. I remember reading about that more than once. If he wanted to make a couple of bucks, he should've stayed in Soil. Because death metal is nobody's meal ticket. So hopefully he's really into it.

Sonic Youth = totally pop

Tony Fredianelli of Third Eye Blind was in a thrash metal band called Apocrypha. Check out both songs, and you be the judge on which band is better

http://www.youtube.com/watch?v=AOf8p6lPh6Q

http://www.youtube.com/watch?v=ohEBJZcngRw

From guitar GOD to WTF