Smaller than Spotify, but still with a sizeable catalogue (13 million+ as of 2014). Slacker free allows you to search for specific artists and ‘start a station’ (will play tracks from that artist/similar artists), or listen to a general genre station. Specific songs can be favorited to locate later from your account (but account is not required to stream songs); however, selecting favorited songs will launch a related station, not play that specific song. Up to 6 tracks can be skipped per station. Premium account required for offline listening. Ads are present in free version.  

App available for Android, iOS and Windows mobile. Browser version also available.