The map symbols for Templar Lairs are actually that of the Knights Hospitallier.
Ezio Auditore da Firenze loosely translates to "like an eagle, auditor of/from Firenze". Auditor derives from the family profession in banking; Firenze is the Italian version of Florence, Ezio's birthplace.
The cars in Abstergo's underground parking lot are from the video game Driver: Parallel Lines (2006).
When you first meet Ezio's uncle, Mario, he says, "It's a me, Mario!", a reference to Super Mario Bros.
If you go to the carnival in Venice, you can vaguely hear the same music that plays during the third class party in Titanic (1997).