The singing bassists thread gave me the idea. I can think of four off the top of my head:
Phil Collins (when he was still with Genesis right after Peter Gabriel left)
Levon Helm from The Band
The lead singer of Night Ranger, whose name escapes me
Sheila E.
Robert Wyatt of Soft Machine did all their vocals (once he left the group, they played only instrumentals). He later did vocals for Matching Mole and had a successful solo career, including a British hit single with “I’m a Believer” (NoClueBoy, thanks for the reminder).