DVDRs. I have 120 GBs of space of my second HD reserved for anime, but I prefer to keep it at most 50% filled (and so I never need to defrag it). And partly because of that I burn series right when they are finished, or half finished (like 13 and 13 episodes of a 26 episodes series). So, I don't actually archive any series on HD at all. They are all just waiting for to be burned.

And of course I have some movies and series as commercial DVDs.