"I am your worst nightmare! A nigga with a badge!"
In the fourth movie, Donkey very poorly sings the final line of Whitney Houston's "Greatest Love Of All", a reference to Eddie Murphy's role as the incredibly untalented Randy Watson in Coming to America.
Shrek tells Donkey "You're going the right way for a smacked bottom". Austin Powers said the exact same thing to Madonna in the Beautiful Stranger video.
Boasting about how his bowel movements could kill a donkey
Taking physical abuse first in his backside, then in his gonads.
Blooper: In Shrek 2, when Fiona knocks out Prince Charming with a headbutt, he's seen seconds later perfectly fine throwing the wand to the Fairy Godmother. You could argue that he recovered quickly, but then a few minutes later, he's seen getting up from the headbutt.
Cash Cow Franchise: In the top 10 for "Highest grossing movie franchises of all time" list.
Deleted Scene: One from film 1 has Fiona meeting a Gypsy woman named Bib Fortuna (a reference to the Star Wars character of the same name), who would eventually become the Fairy Godmother. In this version Fiona is always an ogre, and Bib Fortuna gives her a potion that would make her beautiful, but tells her that she will alternate between her human and ogress forms until she finds true love. After that, she is whisked away by her dragon guardian and returned to the tower.
Also That Other Wiki says that the filmmakers intended to show the "Happily Ever After"-Form of Dragon as a pink winged Mare in Shrek 2. They decided against it. 
Franchise Killer: DreamWorks had plans for five movies in the Shrek film series. Shrek was the first animated film to win the newly coined Academy Award for Best Animated Film in 2001, over Monsters, Inc. and Jimmy Neutron Boy Genius. Shrek 2 was nominated for the award and lost to The Incredibles, but it became not just the highest grossing film of 2004 but DreamWorks' most successful film thus far. Shrek the Third though, was financially successful than Shrek 2, got mixed to negative reviews from critics and was not even considered for Best Animated Film of 2007. The lukewarm reception to Shrek the Third led DreamWorks to finish up the well under development Shrek Goes Fourth, which was retitled Shrek Forever After and became the de facto final movie, ending the series at four. Shrek Pleads the Fifth, the fifth movie, was canceled and turned into a prequel film about Puss In Boots.
One of the major selling points in the Mexican dub was the voice of famous comedian Eugenio Derbez as Donkey. While Humberto Velez was Farquadd.
In the Spanish dub, Shrek and Donkey were voiced by Juan Antonio Muñoz and José Mota, the members of the (now separated) popular Spanish comedy duo Cruz y Raya, which counts as this, Celebrity Voice Actor and Relationship Voice Actor at the same time.
Prince Charming as the fox in the first Narnia film.
Mike Myers' voice for Shrek greatly resembles the voice he used for another Scottish character he played: Fat Bastard from the Austin Powers movies (or Andy Grey from the short-lived Saturday Night Live sketch, "Scottish Soccer Hooligan Weekly")
In addition to voicing Shrek, Mike Myers also voices one of the Three Blind Mice in the first movie.
Captain Hook sings songs by Tom Waits and Nick Cave. It's pretty noticeable since both artists have very different voices.
The Other Marty: Chris Farley was attached to Shrek early in its development, but after his death (and many story changes) the role went to Mike Myers, who then performed an odd same-actor version of this trope, switching to a Scottish accent partway through and re-recording already-done lines.
The original script had Princess Fiona born an ogre to the late King and Queen of Duloc. They had her locked in the tower under the lie that she was "of such rare beauty" she was kidnapped. They died, and the kingdom was left under the rule of an ambitious regent (implied to be Farquaad). When she 'became of age' to ascend the throne she escaped the tower and encounters a witch named Bib Fortuna (a reference to the Star Wars character of the same name) - who has narrated the entire sequence through her tarot cards. She gave Fiona a potion which would make her beautiful; but Fortuna warns her the potion has a side effect - she will change between her human and ogress form until she finds her true love. Later she was whisked away by her dragon guardian and returned to the tower. This storyline was not adapted to keep the story simple, but is mentioned through the "witch" Fiona tells Donkey about.
Chris Farley was originally cast as Shrek and even recorded part of the dialogue. When Farley died, another SNL cast member got his part (Mike Myers).
Paul McCartney originally was in talks to voice Rumplestiltskin, even going as far as having a musical number for him planned with a song written by McCartney himself.