[jtmckinley]

Hmm, well, my definition of prog is broader than most folks, for example I consider the The Beatles, The Who, Led Zeppelin and Tears For Fears to be progressive albeit pop at the same time, so is some XTC. Some interesting picks by other posts, but here's a couple more to consider:

Dale mentioned Mel Torme, another Jazz singer that coulda been great with a prog band (or any band really, and was) is Al Jarreau, check out his cover of the Chick Corea classic Spain.

How 'bout Joni Mitchell? She can (and did) sing pretty much anything from rock to folk to jazz.

And then there's CSN, those three guys (and Neil to some extent too) were sorta prog at least in the early days, check out Guinnevere off the CSN record, it compares to Yes for example vocal-wise IMHO, the mix is less airy and the vocal pitches lower, but kinda similar in the vocal department.

I could prolly go on and on, but I'll leave it there for now.