Shinobi Life is a shoujo manga by Shoko Conami. It's about a ninja named Kagetora who is the sworn protector of a princess. He falls into a lake and is transported to the present day where he finds Beni, a girl who looks exactly like the princess he was sworn to protect.
I read this manga back before it was licensed and enjoyed it. It's currently being released in America by Tokyopop.
Image Archive (.zip)