i'm rather partial to trip-hop, so my picks are as follows:
Skye Edwards of
Morcheeba
Dido for her solo stuff and her work with
Faithless
Tracey Thorn of
Everything But the Girl and on the mind-blowing song Protection from
Massive Attack (didn't really care for her solo stuff though...)
Shara Nelson who also worked a bit with
Massive Attack (Unfinished Sympathy, Safe From Harm)
Elizabeth Fraser more vocals for
Massive Attack (Teardrop)
Costanza Francavilla who worked with
Tricky on his album Vulnerable
and of course the many talented vocalists who have worked with
Zero 7
but my absolute favorite has to be:
Geike Arnaert of
Hooverphonic, who recently left the band (

) but is now recording solo stuff (

)
also worth mentioning for their unusual (yet still beautiful) voices are:
Alison Shaw of
Cranes
Melora Creager of
Rasputina
and last but not least, the wonderfully talented
Björk Guðmundsdóttir 