I'm using http://www.music-map.com/
since a few months and I'm surprised nobody mentionned this. I have discovered dozens of artists I really like using this tool.
You just input an artist's name and it will give you a map of other artists. The nearest to the centre are the most similar artists, and as you go farther it becomes more and more different.
I simply search for the artists I find on youtube and listen a bit to how they sound. I only go to the middle of the map usually, after it gets a bit too far from the artist usually.