Replayify - Create playlists from your most played Spotify tracks
Magic Playlist - Get the playlist of your dreams based on a song
SpotiMy - Play/Pause and Next/Previous functions for Spotify directly on your Menu Bar!
Spotalike - Spotify playlist with similar songs, according to Last.fm
Muzzy - it’s a tiny and gorgeous iTunes companion that lives on your menu bar. For Mac OSX, its free!
Playlist Machinery - Tools that help you create & organize your Spotify playlists