Songstack - Swipe to discover new music for Spotify
mixmello - mixmello is a free, open source tool to create remixed versions of your favourite playlists.
Incredibox - Express your musicality, pump it up and chill
Music Walker - Explore and play (via YouTube) songs similar to a seed song.
Musicroamer - A visual Pandora for discovering new music
stats.fm - With the click of a button you'll be logged with your Spotify account and you'll instantly gain access to a valhalla of cool stats and insights.