Sankarea OVA
Two OVA episodes are bundled as DVDs with each of volumes 6 and 7 of the manga.
The first OVA is a prequel story written by the manga's author. Before the beginning of the main story, Rea and Chihiro have already met at a certain place: an outdoor hot-spring bath.
The second OVA takes place after the series (effectively episode 14). The Furuya family finds a mysterious young girl hiding under the household temple.