We mentioned this in the prequel list, but as they're the films that started it all 1138 needs to mentioned. 1138 is one of the most prevalent easter eggs in Star Wars, not just popping up in all six films but also a plethora of games and other tie-in material. A little nod to George Lucas' first feature film, THX 1138, over time it's become a popular wink for all manner of directors.
Initially Lucas was going to go overboard with references. In Star Wars it's most prominent when spoken by Luke when trying to rescue the Princess ("prisoner transfer from cell block 1138"), but is also seen on a console behind R2-D2 and C-3PO on the Death Star. And there could have been another one; in the original script the Stormtrooper whose armour Luke borrows is codenamed THX 1138, not TK-421.
Over the course of the trilogy the references became harder to spot, with it cryptically delivered in Empire as command from General Rieekan ("send Rogues 10 and 11 to Station 3-8") and near impossible to make out in Jedi (its on the side of Leia's helmet when she sneaks into Jabba's Palace).