You know them. Kinda. They’re “that other guy” that was in that band with the super-famous person before they were excised like the moldy corner on a block of cheese.
Garfunkel. Oates. That other guy in Wham! One of Destiny’s Children not named Beyoncé. One of the guys in U2 that didn’t get a cool name.
Who would you want your name forever attached to, except most people only know your name as “that other guy”?
Not that I’m disparaging these people in any way.
Frankly, personally I feel like it’s the ideal way to be famous. You get your share of the spotlight and glory for as long as it lasts, then you can walk away, collecting massive royalties while your former partner has to deal with all the attention, or remain in the background and still get to play and tour and collect big paydays while Bono has to go do photo shoots and deal with the press 24/7.
Great idea for a thread!
Easy for me:
Aerosmith, Brad Whitford - Joe Perry is the only guitarist who gets discussed, who is half of the Toxic Twins with Steve Tyler, etc. But BW is a great player - I think he plays the opening riffs to songs like Mama Kin and Same Old Song and Dance.
AC/DC, **Malcolm Young **- Angus who? 
Rolling Stones, Ron Wood - you’re second fiddle to Keef but you can still play guitar like Ron Wood. He is just a brilliant player. Cool by me!
I’d go with Hubert Sumlin, longtime guitar player behind blues legend Howlin’ Wolf.
Steve Vai
That other guy in Wham is married to the dark haired girl in Bananarama. I’d take all of that, thank you very much.
I wouldn’t like to be the third guy in Bros who wasn’t a Goss. Poor kid.
If I could somehow be the brother in The Corrs while not actually being their brother, I would die happy.
John Paul Jones, bassist/keyboardist of Led Zeppelin.
Plant and Page get all the ink, and Jones has been known to have felt overlooked from time to time even by the both of them.
But Jones is a hugely talented multi-instrumentalist (From Wiki: organ, guitar, koto, lap steel guitars, mandolin, autoharp, violin, ukulele, sitar, cello, continuum and recorder) who’s had a remarkable in-demand career in music since Zeppelin disbanded following the death of John Bonham in 1980. Many amazing Led Zeppelin songs would be far less majestic if not for his contributions.
Bernie Taupin, Elton John’s lyricist. From what I understand he just writes his lyrics in his English home and dashes them off to Elton to write music. Elton has always been generous in his praise for Taupin, whether it be in interviews or in album packaging credit. So Bernie gets all the fame and money of being a big time musician without having to worry about touring/the tabloids/overzealous fans.
Charlie Watts is the coolest man on the planet.
Vanilli.
Great choice, and it’s clear that Page and Planr have not included him in their somewhat innerer circle. But anyone who can write tiffs like Misty Mountain Hop and Trampled Under Foot - yeah, that would be cool.
Beat me to it. That’s the first to come to mind for me, and for all the reasons stated.
Chris Wood of Traffic.
Probably Michael Anthony, most famous for being the bassist in Van Halen through 2002. I think some of his credits/rights for a couple albums (Van Halen and 1984) were bought out by the band around 2002, but he’s still collecting, I believe, on the rest of his work with them. And he seems like a genuinely great guy.
George Harrison.
No explanation needed.
I’ve always thought the bass in Led Zeppelin songs was too low in the mix and the electric guitar too high, but then Jimmy Page (the guitarist) did produce the band’s records.
Bill Wyman or Marc Perlman
…but I can’t play bass. 
Jack Sonni, second guitarist for Dire Straits during the Brothers in Arms era. Studied literature, had his music career, raised a family, now doing some writing. When I see him in old concert videos, he just looks like he’s having the time of his life.
One of the Supremes.
The third guy in Bros was the only one of three to make any money from the band. he sued the other two after they split and won a bucket load of money.