A few of mine (I'm mostly just going to go into their voices- their albums are vocal-centric anyway).
Dido (full name: Dido Florian Cloud de Bounevialle O'Malley Armstrong- rather a mouthful) The words for her are: intimate, natural, and emotionally involved. Generally of lower intensity, and more breathy. She sounds like she's singing just for you. She has a tendency to be a bit melancholy. The recording quality of her first album wasn't that great, but after that the production values went through the roof. She also started her career with Eminem (the rapper), but you'd never know it. My metaphor for what would happen if you put a roadblock in front of them: Dido would sigh and say "not this again," and wait for it to clear. From England
Jem: A nice contrast to Dido- the words for Jem are: dark, powerful, confident. Where Dido is more delicate, Jem is has much more power (that's her signature). A bit pop-y (She has sung for some recent movies and prime-time TV shows). She has a much lower reach than most female singers I've heard. Roadbock metaphor: Jem would slowly steamroll over the barrier after warning you that she won't wait for anyone. From Wales.
Karen Peles: Every syllable is deliberate, but with tons of energy behind it. Roadblock metaphor: Peles doesn't even look at roadblocks, or signs, or anything. She smashes through at full speed and never looks back, as if it didn't exist, and warns nobody. She's not singing for you... she's singing because she wants to, and you just happen to be there. A true Sabra, and not well known in the US. From Israel.
Imogen Heap:I've never heard someone so flexible, versatile, and yet so in control. She can be soft and intimate, dark and powerful, or energetic and bright. She conveys many emotions. Her annunciation is always clear, precise... I'd consider her to be between Dido and Jem, but much more versatile than either. Every song is different, and nobody sings quite like her. Nobody's mixing is quite like hers. Note: she shared a producer with Jem for a time (I'm unsure if it was simultaneous). She is closely involved in her recording, as a bit of a nerd, so the technical aspects are stellar. Roadblock: She'd annoyedly storm up and demand to know why it's there, wait impatiently for a bit, and then walk around, head high, and when nobody's looking, she'd make some rude gesture back. From England.
Vanessa Carlton: Innocent, Intimate, trusting, emotional, a bit melancholy. She's singing just for you, and she sings with the attitude where she feels like she can trust you with her secrets (did that make any sense?). While similar to Dido in some respects, Carlton sounds much younger and more "innocent." Note that often she seems to sing in metaphor. Roadblock: She'd sit down nearby and patiently wait, feeling quite sad inside. American.