Solo Singers-
Matthew West
Elton John
Tarja (She was from Nightwish, but isn't anymore a member ofc you probably know)
Singers from Bands-
Sharon den Adel from Within Temptation
Anette Olzon from Nightwish
Ben Burnley from Breaking Benjamin
Christian Liljegren from Golden Resurrection (Thanks Nick XD)
Michael Barnes from Red
Pretty much more, but these are my most favourite ._.