Angel Sanctuary is a little known (at least in the US) manga series that recently got turned into an anime and I believe was just released a few months ago in Japan. I have the OST, so it's definitely been released somewhere. I don't know too much about it except the basic storyline, but what attracted me more was the illustrations rather than the plot itself. As it is, the plot is somewhat disturbing, since incest is involved (yuck).

Basically, a war between the angels of heaven and of hell breaks out, and Alexiel, a fallen angel, leads the demons of hell against her brother and the angels of heaven. She is defeated, and her brother punishes her by turning her into a human, which might be bad enough, only she is doomed to be reborn over and over, and all of her lives are to end in tragedy. Setsuna, Alexiel's present form, is a teenage boy in love with his sister Sara. The demons, however, have realized that Alexiel is sleeping within Setsuna, and appear as humans to try to wake her up. In the meantime, her brother is looking for her at the same time.

It's a pretty dark series, from what little I know about it. Hopefully the anime will be taken up by some fansubbers so I can actually watch it and understand it better. Till that happens, there won't be any updates on this series really at all.