 If you liked
R.O.D: Read or Die
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 ...then you might like
Black★Rock Shooter (OVA)
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At first this two series seem very different, but in the last episode of R.O.D their similarity can be seen. They both have action, and also deals about two friends (in R.O.D partially).
One of then turns against the other for unknown reasons (Yomi and Nancy). And also in both of then Yomiko and Mato try to save their friends, but they also have to fight them (not so much Yomiko VS Nancy as Dead Master VS Black Rock Shooter). Finally they are both very dramatic and sad at the end.