Clannad. Man, I remember watching the anime, and crying like a little bitch for DAYS. Shook me up completely. What a timeless classic. Too bad I don't have the heart in me to play this, I don't think I can endure the rollercoaster of emotions I experienced all those years ago.
If you end up here accidentally, do also give the anime a try. Its so incredibly beautiful. Its not all gloom; it has some really funny moments. But it will definitely make you think hard about your life.