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
Songstack - Swipe to discover new music for Spotify
Remix of the Century - A mix of every #1 from the Billboard charts since 1890
The Rick Astley Remixer - Remix your favourite songs in your browser.