Free Music allows you to download millions of free songs from website that legitimately give you that option. All the music is licensed as "free to use", so feel free to download as many songs as you want! You're ok as long as you don't try to sell it!
As for music source, popular sources like LastFm are used, as well as Jamendo, Free Music Archive or Live Music Archive !
Note: you won't find music that needs license here. This is not a piracy app!