Kimi ni Todoke by SHIINA Karuho deserves to be mentioned. As the story progresses the focus shifts from the main couple to their friends love life, until the screen time is almost equally divided between the multiple relationships.

EDIT: Sorry, just realised that it has already been suggested. Well, that's what you get for not reading previous posts properly sigh

[color=#8A795D]"Stories are what death thinks he puts an end to.
He can't understand that they end in him, but they don't end with him."

• Ursula K. Le Guin, Gifts[/color]

To be savoured:

• Blood Alone by TAKANO Masayuki
• Otoyomegatari by MORI Kaoru
• Gangsta. by Kohske
• Seishun Kouryakuhon by AKIZUKI Sorata

H2O - 3 couples in 1 <3
Love So Life
Ingenuo - The side-pairing even got their own manhua: Steel Rose
You're So Cool - The side-pairing is shounen-ai, sorry ~ still, their story is adorable! (and I'm not even fond of yaoi)
Beauty is the Beast
Special A
The Wallflower
Biyaku Café
Nijiiro Days (MIZUNO Minami)
Lovely Complex

"But if I see your eyes once more... I'll be able to feel that it is you..."