Last.fm - The world's largest online music service. Listen online, find out more about your favourite artists, and get music recommendations, only at Last.fm
Gnoosic - Even if you don't know what you are looking for - gnod will find it.
Pandora - Pandora Radio is a free (ad-supported) internet radio founded in 2000.
LivePlasma - Discovery engine map for books, music, and movies
Deezer - Deezer is a music streaming app created in France. It is available in 180 counties and gets 16 million users a month. 6 million of the users have paid subscriptions. Read more about Deezer.
Music Walker - Explore and play (via YouTube) songs similar to a seed song.