mixmello - mixmello is a free, open source tool to create remixed versions of your favourite playlists.
Songstack - Swipe to discover new music for Spotify
Incredibox - Express your musicality, pump it up and chill
Musicroamer - A visual Pandora for discovering new music
Music Walker - Explore and play (via YouTube) songs similar to a seed song.
Remix of the Century - A mix of every #1 from the Billboard charts since 1890