Oh wow, we actually didn't have an Air thread.
Air is originally a visual novel from KEY, the same guys who did Clannad and After Story - But it's much less known than those two.
Still, it's quite sad. Heart warming and sad at the same time, and definitely something you'd watch for the story of life, rather than going to look for sad things or some other silliness.
Spawned some amazing AMVs. There was one other particularly notable one, but it was removed from youtube =x
Excellent series, it makes me nostalgic.
... Gao.