"This free music player lets you plot your mood on a spectrum and then creates a chart of songs to fit your mood. You can further narrow your results by genre, year, and whether the tunes are hits or not. Watch Musicovery plot connections between artists and songs...."

From downloadsquad:
Musicovery is a site where you can discover new music based on how you're feeling...Songs are represented in color and are connected by their relationship to one another based on tempo, and positive or negative lyrics. No log-in is required to start testing it out. Simply pick out your music category: rock, latin, rap, disco, soul, etc. Then go to the mood/energy matrix and click where you want to start. Music based on your mood preference will appear and you can click on different songs to explore the sound. ... the site is intuitive. You select a song based on your mood, and then like magic, the music just plays."

A paid account starts at $4, features high-fi, allows you to save your favorite songs and has no ads