Air and Aishiteruze Baby are both about people having to take responsibility for a little girl. There are choices made that effect their whole lives and they have to deal with their individual decisions and grow up on their own, even though the situations are different in both series the development of the main male character through everything he has to go through is the same. Both Kunisaki and Kippei (Air and AB respectively) have to go on their own journey to find themselves with the little girls being the vessel for them to do that.
Some time before Otonashi made his way into the lives of Yuri and the gang, and prior to the creation of the SSS, Hinata is involved in a car crash and wakes up in life after death. Surrounded by strange classmates in a strange school, he quickly meets Yuri and, after giving her the nickname Yurippe, the two begin to plan out their fight against god. That is, if they can figure out how to deal with the student council president...
If a comedic drama mixed with a heart-wrenching story is what you are looking for Clannad and Air are the series you will want to check out. There is an exception in that Clannad has more comedy in it whereas Air is more drama related.
Both Air and Da Capo are harem in genre. They also both deal with issues one would have to face in a high school setting. In each series the main hero is trying to find out what is going on around him and how everything impacts the main heroine, the one that he loves. He also helps the other girls that like him.
Tohru Honda might just be the unluckiest girl around. After a series of unfortunate events Tohru has no place to call home - that is, until the wealthy yet cursed Sohma family offer to take her in, in exchange for performing the household chores. However, soon Tohru discovers that there's much more to the Sohma family than meets the eye. Though shocked and surprised, Tohru promises to keep their secrets; but will the Sohma family approve of an outsider knowing who they really are?