The one I like is Every Noise at Once: http://everynoise.com/engenremap.html
It links to Spotify to allow you to listen to genres as you click on them and also generates two different playlists which you can import to your own Spotify account; 'Pulse' (most listened to in that genre) and 'Edge' (new discoveries in that genre)
Also worth a mention is the gnod 'System of Discovery' http://www.gnod.com/
Not just music but also films, books, etc. Has a similar Music Map (by artists this time) http://www.music-map.com/
but more interestingly, provides recommendations based on 'your 3 favourite bands': http://www.gnoosic.com/