I am notoriously behind what is popular in music, so I'm sure I'm the last person to get into the following:
My sister turned me on to Lily Allen...she is just my kind of pop music (I'm not usually a fan of pop but her music makes me feel good). Perfect mood lifter, and her cockney accent is adorable. Good for working and exercising too.
Otherwise, I've been absorbed by the bizarre, lovely, insane music of Regina Spektor. This video is very mainstream and calm, but much of her other music is wild, untamed brilliance that I adore more. You can hear echoes of Tori Amos, even Joni Mitchell in a lot of her work, but then she's got her own thing going on completely.