Well, as a bassist who sings some back-up, I know it is difficult to sing and play bass... but here's a few more: Roger Waters in Pink Floyd (although he wasn't the only singer); Rick Danko in The Band (again, not the only singer); Chris Squire sang really odd back-up parts in Yes, which had NO obvious relation to the main vocals and were probably more difficult than just singing a "normal" song and playing a "normal" bass part; Greg Lake in ELP; Ronnie Lane in The Faces (sang lead on some songs, but when you're in a band with Rod Stewart you tend to be over-looked when you sing!); Laura Love (even though most likely none of you have heard of her); Me'Shell Ndegeocello; Benjamin Orr in The Cars (shared lead vocals with Ric Ocasek).
Those are the ones I can think of that haven't already been mentioned. Oh! And Steve Harris in Iron Maiden sings pretty much every single song when they play live, but he doesn't have a mic! But he writes most of the songs, so he is singing his heart out silently as he plays.