Magic Playlist - Get the playlist of your dreams based on a song
Plexamp - A small music player with a number of unique features
Spotalike - Spotify playlist with similar songs, according to Last.fm
Vinyls - Vinyls is a music player that lets you listen to your music without any complicated setup.
Playlist Machinery - Tools that help you create & organize your Spotify playlists
SoundWire - SoundWire does audio mirroring (audio cast). You can use any music player on your PC or laptop like Spotify, YouTube, or iTunes and stream low-latency live sound over WiFi directly to your Android device.