A film from Studio Ghibli (Spirited Away), Grave of the Fireflies is a film about a teenage boy and his four year old sister, attempting to survive in Japan in 1945. Apparently it is the second most popular film in Japan, after the Seven Samurai. I watched it this weekend.
A tragic film that will rip your heart out. I have never cried so much. A strongly anti-war film.
A good feel for the power of this film can be gained from the imdb comments, which I've been reading over lunch.