Trivia: The hair salon marge goes to is called Hairy Shearers, a reference to Harry Shearer.
Cape Feare - S5-E2
Trivia: Bob's prison number is A113. This number is often used as by alumni of California Institute of the Arts (most famously by Brad Bird, in his Pixar films and episodes of the Simpsons he was involved in, such as this one) - it's the number of one of the classrooms.
The Principal and the Pauper - S9-E2
Trivia: Martin Sheen was chosen to voice the "real" Seymour Skinner because the producers had admired his role in Apocalypse Now, and thought his voice would be appropriate for a Vietnam veteran.
Cubs FanTrivia: Paul McCartney agreed to guest star on the condition that Lisa's conversion to vegetarianism remain permanent. This is one of the few points of continuity that has been strictly adhered to.
Cubs FanTree House of Horror IV: The Simpson's Halloween Special IV - S5-E5
Trivia: When Marge asks Bart to explain how scary the episode is, she mentions the "War of the Worlds" broadcast on NPR. Narrated by Orson Welles, that radio serial was so chilling, a number of listeners actually believed Earth was under attack.
Cubs FanTree House of Horror XII - S13-E1
Trivia: In "House of Whacks", the murderous house is disengaged when its British charm unit is removed; Pierce Brosnan, the personality whom the family had chosen for the house, is actually Irish.
Cubs FanTrivia: One of the sliest jokes in the series' entire run is the name of talk-show host Opal's boyfriend, Straightman. Steadman Graham, the longtime beau of Oprah Winfrey (on whom Opal is based), is rumored to be gay.
Trivia: When Homer sees Moe as the little bird in the cuckoo clock, it keeps repeating "cuckold" instead of its customary cuckoo call. Homer then asks, "Whats a cuckold?" The term is generally applied to a husband whose wife is sleeping with other men. Perhaps fitting, as Homer suspects her of having an affair. (Source).
Lummie
Trivia: This episode contains pictures of Peter Griffin from Family Guy & Stan Smith from American Dad.
Dan23
When You Dish Upon a Star - S10-E5
Trivia: Another thing "The Simpsons" predicted: Towards the end of this episode, a sign outside of a film studio shows the 20th Century Fox logo with "A Division of Walt Disney Co" written under it. Nineteen years after this episode aired, Disney purchased many assets from 20th/21st Century Fox, including "The Simpsons."
Phaneron
Trivia: Dan Castellaneta and Hank Azaria both voice about ten regular characters, with Azaria contributing another ten recurring, and Harry Shearer voices twelve regulars.
Cubs FanSideshow Bob's Last Gleaming - S7-E9
Trivia: When we are first introduced to Sideshow Bob, he is building a ship out of sticks. Look at his name tag. It says A-113 on it. A-113 is the classroom number used by character animation students at CalArts, frequently used as an inside joke by graduates, right the way up to Pixar movies.
Trivia: In a recent episode (I saw it when it premiered on 5/17/09) the Odgenvillians move into Springfeild, then are forced out. When Lenny and Carl go on the chair attached to balloons, you can see the house for Pixar's latest film, Up, in the background.
How I Spent My Strummer Vacation - S14-E2
Trivia: At the Rolling Stones' fantasy camp, Mick Jagger's diploma from the London School of Economics is on the wall in his office.
Cubs FanTrivia: Marge's hairstyle in this episode is the Rachel, popularized by Jennifer Aniston on Friends.
Phaneron

Trivia: In the opening sequence post Feb 2009, Marge can be seen placing items on the checkout. These include the detergent Mr Sparkle from Season 8 Episode 22, In Marge We Trust, and some Tomacco juice from Season 11 Episode 5, E-I-E-I - (annoyed Grunt). (00:00:25)
Ssiscool
Dude, Where's My Ranch? - S14-E18
Trivia: At the beginning of this episode, Homer writes his own Christmas carol after being barred from performing traditional carols because of copyright issues. Around seven years after this episode aired, the musical artist Usher was ironically accused of plagiarizing one of the tunes Homer was working on with his song "OMG."
Phaneron
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.