To play music according to your mood, you can check out AUPEO!, Moodstream, Musicovery and some others as mentioned before.
The following will show you a new one suggested by David Pratt — Muzikool, which is similar to the historical MeraMood, since all the music is from YouTube.
On the Muzikool site, you can make a playlist of Excited, Happy, Pleasant, Calm, Blues, Sad, Upset and/or Wild moods, select a decade (such as 70s, 80s, 90s, etc.), and click on the "Go" button, then you can watch and listen to the music videos on the right side.
When choosing your moods, you can drag and drop the green point to the mood you would like to start, move the red point to the mood you would like to end, and you can also adjust the two grey points to make a path from the green point to the red point as your playlist.
When listening to a song, you can favorite it, ban it, and correct it with the right mood, besides, you can also read its artists bio from Wikipedia, comment on it with your Facebook account, and share it on Twitter or Facebook with the original link to YouTube.
In fact, besides playing music according to moods, you can also make a playlist of Britney Spears, Justin Bieber, Katy Perry, Lady Gaga, Rihanna and/or Taylor Swift.
Cool, right? But Muzikool is not available for you to select the video quality, and what is more, it is still in beta testing and you need ask for an invite to join in, good luck!