Using "Stumble Upon", one of the Firefox add-ons, I just found this site: Musicovery
You choose your style (disco, classical, jazz, cinema, gospel, rap, pop, etc) and your mood (dark, positive, quiet or energetic). The page immediately proposes you with a trail of topics, just click on what you want to listen.
For example I asked for classical/quiet and was proposed with Chopin's Preludes, several Debussy and Ravel's pieces, Beethoven's Appassionata, Prokoviev, Saint-Saëns, and else...
A very good way to listen to the music fitting your mood whilst working :o)
NB - the page is in French but it's not difficult to understand how it works LOL