I don’t know if it was already mentionned earlier in here but Rewrite Side-R seriously had lots of references. There was a Dango Daikazoku logo on that box Chihaya held when she first appeared, and Kotarou found a magazine titled “Masato’s muscles or something” when he looked for Kotori in the forest (in the beginning) and remember that guy who controlled a puppet and made Akane spit the juice she was drinking?, they made sure that he looked exactly like Yukito, the puppet too. There’s also an OMAKE in one of the volumes where the heroines wore the other Key uniforms. If I remember correctly, it was like this: Kanon’s for Kotori and Air’s for Chihaya (or the opposite, I don’t remember that well), Clannad’s for Shizuru, Little Busters for Lucia (she wore it exactly like Kurugaya just so you know) and Angel Beats’s for Akane (no, not the school’s but the after life battle front or whatever is its name). There were other references but I have to read it again to find them.