Clannad is (A visual novel) by KEY and animated by KyoAni (Lucky Star, Haruhi). Considered the greatest romance anime of all time (Though not by me, which doesn't stop me loving it to pieces regardless.)
You may be thinking of Chobits by CLAMP, they're the most well known mangaka group and also made Cardcaptor Sakura (A masterpiece) and Magical Knight Rayearth.