Aside from the obvious (Charlie Dominici and James LaBrie):
Steve Stone sang after almost having been given the job as the new singer of Dream Theater, a show where Dream Theater opened for Marillion. The show has infamously been dubbed “screaming for me long beach” due to Steve Stone constantly yelling that famous Bruce Dickinson line – except they weren’t at Long Beach.
Mike Portnoy had the dubious privilege of singing the end of “Surrounded” after James walked off the stage at one of their 1993 England shows. He also sang the beginning of “The Killing Hand” at Toad’s Place in New Haven, CT in December of 1996, when James “…got caught up backstage”. Mike has also done the first few lines of “Take the Time” at nearly every show on the FII tour.
Other instances of singers other than James singing with the band:
– Mike Portnoy picked up lead vocal duties during the first verse of “Anna Lee” at the February 26th, 1998 show in Brussels Belgium. The verse was followed by “Where the f— is James!?”
– In February 2000, when James left the stage under threat of arrest from the fire marshall, Mike sang most of “The Spirit Carries On” with the entire crowd singing along.
– On the 2000 Christmas CD, there is a version of Black Sabbath’s “War Pigs” with Greg Chick (who was pulled up from the crowd) on vocals (see attached photo).
– Chris Jericho (a.k.a. Fozzy’s Mongoose McQueen) sang with the band on stage for one song in Canada in early 2002.
– Bruce Dickinson has sang with the band on a number of occasions, including a BBC Sessions-style cover of Deep Purple’s “Perfect Strangers”, and in Los Angeles on 5/18/98 when he sang “The Trooper”, “Where Eagle’s Dare” and “Killers” (all Iron Maiden) with DT. That show is available through Ytsejam Records as an official bootleg.
– Also at that show, Ray Alder sang “Pull Me Under”.
I’m not sure whether these count or not, but anyway…
-Russell Allen (Symphony X, Adrenaline Mob) and Burton C. Bell (Fear Factory) joined Dream Theater onstage while they performed Cemetery Gates (Pantera) during the Gigantour (2005);
-Mikael Åkerfeldt went onstage to sing the “day after day and night after night” part of A Nightmare to Remember during the band’s performance at Wembley Arena on 10/10/2009.