Vongola
G = Giotto
27 = Tsunayoshi Sawada
59 = Gokudera Hayato
80 = Yamamoto Takeshi
18 = Hibari Kyouya
L = Lambo
84 = Basil
33 = Ryohei

Gabaleno
D/Boss = Dino

Kokuyo
594 = Kokuyo
493/49 = Chikusa
69 = Mukuro
96 = Chrome

Acrobaleno
R = Reborn
56 = Colonello
M = Marmon

Varia
3/X = Xanxus
39/S = Squalo
B = Belphegor

Millefiore
100 = Byakuran
48 = Spanner

Other
86 = Haru
87 = Hana

Example:
2769 = Tsuna x Mukuro
*The set of numbers which comes first shows who's the seme.


Thanks to Chikusa Kakimoto.
Source: Livejournal Community