It depends on what I'm trying to get inspiration for. The main character of Blue Star Rebellion? I'll listen to Billy Joel, James Taylor, Pink Floyd... Or Simon and Garfunkel, if he's supposed to be feeling quiet, or sad.
The main romance for my other eventual project? Lots and lots of Jason Mraz. The boy is a guitarist, and would totally sing love songs.
Magic practice and mage fight music? (Also for my other project.) Pop-ish celtic music with a driving rhythm.
I also absolutely love Pat Metheny, and his music has a lovely ambience which fits loads of different characters and scenarios.