I've posted about [adult swim]'s Ghostly Swim compilations before, but was unaware of their much larger catalog of freely available music. Today's recommendation: The Metal Swim compilation.
But honestly all their stuff is worth checking out. The Singles 2015 collection looks excellent as well. I am slightly flabbergasted I haven't seen this before. (Heads up: Some of the album pages require flash to play or even download the music.)