Kimi ga Nozomu Eiene is both the best eroge made IMO (17 endings I believe, many different routes, a fuckload of girls, melodrama, lots of sex) and also the best anime adaption of an eroge.
Don't listen to these anons. The coma girl had the hots for the male lead, so her best friend offered to become friends with him so that she could get them together (because coma girl is shy). This plan works -- the male lead decides to start dating the coma girl, but with a twist; both he and the best friend really hit it off, but unable to confess his feelings, the male lead decides to give up on her and stay with the coma girl who loves him.
But the best friend -- she decides NOT to give up. One day, the male lead is going to go meet coma girl for a date; the best friend intercepts him on the street, and in order to get her hooks into him, says that it's her birthday and demands to be bought something. He does; a fairly expensive ring, but this makes him really late for his date.
Being late, coma girl is struck by a car and is sent into a coma for several years. During this time, the best friend, empowered by the dark power of her One True ring, is able to obtain the male lead and they fuck constantly up until the day the coma girl emerges from her coma.