Bandcamp, for those artists who use it. MP3 is the default format but you can choose the quality, and there are audiophile (FLAC) and geek (Vorbis) options. The artist sets the minimum price (as low as free) but you can pay more if you want to.
I hate it when I can only legally download music from iTunes or Amazon. If I ruled the world I would force everyone selling music in those stores to also make it available on Bandcamp.