I found links to the GrooveShark API on my site. Interesting API calls that seem to do what you want include ...
addUserLibrarySongs
getUserPlaylists
subscribePlaylist
getUserLibrarySongs
getUserFavoriteSongs
addUserFavoriteSong
getPlaylistSongs
getPlaylistInfo
setPlaylistSongs
createPlaylist
getDoesSongExist
getArtistsInfo
getAlbumsInfo
Search
getPlaylistSearchResults
getSongSearchResults
getSongURLFromSongID
getPlaylistURLFromPlaylistID
getTrialInfo
source
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