Derek Holt - Climax Blues Band. Wrote, sang their hit "I Love You". I think he played piano on that cut. He plays guitar, too.
Paul Rodgers - Free and Bad Company. He started out on bass, though he never played bass live or on any recordings. Interesting tidbit, but I don't think it counts on this list, really.
Kenny Rogers. The First Edition, solo. The Gambler himself used to rock out with a four string before becoming a country crooning megastar.
Tom Evans. Badfinger. Usually sang backup on the hits, but he could hold his own when handling leads. Like the rest of that band, ripped off and died too young.
Boz Burrell. King Crimson, Bad Company. Despite being a competent lead vocalist, Boz was second fiddle to Paul Rodgers. Like Adam Oates was stuck behind Steve Yzerman.
More later, my brain turned to mush. Sorry.