The Secret of Monkey Island In-jokes
From Monkey Island wiki
"I'm selling these fine leather jackets"
When he first meets Carla one of Guybrush's possible dialogue lines is "Hi, I'm selling these fine leather jackets". This is a reference to the game Indiana Jones and the Last Crusade, where Indy uses this line to bluff his way past a German guard in Castle Brunwald. Guybrush can also tell the Fettucini Brothers "I'm selling these fine jackets." The line would return in later games as well.
"Ask me about Loom"
In the Scumm Bar Guybrush can find a pirate named Cobb, wearing a badge reading "ASK ME ABOUT LOOM". If Guybrush does ask him about Loom, Cobb launches into an enthusiastic spiel about its wonderful storytelling and sophisticated features. Not only was Loom the previous adventure game from LucasArts, but Cobb is based on the character Cob, the ugly dungeon keeper from the game.
In The Curse of Monkey Island a skeleton resembling Manny from Grim Fandango has a button that says "Ask me about Grim Fandango, which Guybrush will refuse to pick up ("I don't want people asking me about Grim Fandango").
"I'm Bobbin. Are you my mother?"
The main character of Loom is called Bobbin Threadbare, and the storyline has him searching for his mother, who has been turned into a swan. Guybrush can use this line twice: once after being shot out of the cannon by the Fettucini Brothers, and once while talking to Meathook on Hook Island. Meathook's answer is "No, your mother was a duck."
On the tiny jetty at the back of the Scumm Bar there is a seagull pecking at a fish, which Guybrush can shoo away. There's nothing further in-game to identify the seagull, but the credits at the end include the line "Cobb and Seagull appear courtesy of Loom".