The intro sequence of the show has a bit where Grim's head is ablaze and laughing with a swirly background behind him, referencing the intro to Grim and Evil, back when the series shared a timeslot alongside Evil Con Carne.
The very first episode of the series is a blatant pastiche of The Seventh Seal, except the Grim Reaper is wagering a hamster's life over a game of limbo rather than a knight's life over a game of chess. And the Billy & Mandy version isn't in Swedish.
Their version of Dracula is not only black, but he's based on Redd Foxx.
The episode 'Tricycle of Terror' is likely a parody of Christine, as it involves a tricycle which is haunted, can repair itself after being destroyed, and attacks the enemies of its nerdy owner before being crushed to destruction in a final confrontation.
The Father's Day episode includes an appearance by Country Bears lookalikes.
Someone on the show must have been a fan of Dune, because it gets more shout-outs than you'd expect from a children's show.
One episode was a straight-up parody of God-Emperor of Dune, with Mandy in the Leto II role, Billy in the Duncan Idaho role, and Grim still as Mandy's servant. Spice is replaced with cinnamon, which spice is said to similar to in smell and texture.
In the beauty pageant episode, one of the tests turns out to be gom jabbar.
"Wand of Magic Missile — I'm never gonna use that thing..." said offhandedly by Grim as he rummages through his trunk of magic stuff in the basement. Obviously a reference to 2nd Edition D&D as wands of magic missile in that version of the game really were useless.
The Elemental Plane of Fire briefly appears in one episode.
A subtle one in the Wrath of the Spider Queen special with the dark elves working for Velma Green.
Later in another episode, Skarr's mutation makes him resemble the eponymous alien minions from Pikmin.
He's also a parody of Herr Starr from Preacher, being a bald, one-eyed villain subject to various humiliations throughout the series.
In "Billy and Mandy Vs. the Martians", Grim casts a spell that gets Billy's nose stuck to an alien space ship. When Billy asks him to cast it again to undo it, Grim says he can't because it has a ten minute cooldown.
Not to mention the Martians themselves eerily resemble the ones from Mars Attacks!.
In "The Firebird Sweet", Grim directly references the 1939 movie Drums Along the Mohawk when the group was about to enter a factory via the sewer... then says that he was referring to the director's cut when Mandy questions the authenticity of his claim.
Mandy (off-camera) smells smoke after Grim and Billy accidentally torch the carpet. Mandy tells them that if she finds a scorch in the carpet that "I'm opening up a can of Powerpuff on you two." Also, in the Kids Next Door crossover, Billy answers the door, sees the Kids and says "You're not the Powerpuff Girls!" It's worth noting that The Powerpuff Girls was originally intended to target an older audience. And it was going to be called "The Whoopass Girls". Now take what Mandy said, replace "Powerpuff" with "Whoopass", and it all makes sense.
"Alas, poor Grim! I knew him Billy!" They get it closer to the real thing than most people, who substitute "I knew him well" for "I knew him Horatio", or some other variant.
Sperg tossing Billy's bicycle over a sidewalk cliff, "This is Where the Sidewalk Ends Baby!" The cliff is also a reference to the book's cover.
In one episode, Mandy demands that they leave the beach because she's tired of, among other things, an annoying dog that won't stop biting the back of her swimsuit, a la Coppertone (thankfully, Mandy is wearing a one-piece suit).
In the episode "Crushed," Mandy receives a makeover a la Sandy from Grease, with a heel-crushed lollipop in place of a cigarette.
In the episode "Smarten Up" Billy’s cat Milkshake eats an intelligence enhancing bookworm that was meant for Billy. After Billy complains about never being able to pass a math test now that the worm is gone Milkshake states “…I'm very well acquainted with matters mathematical. I understand equations, both the simple and quadratical.” These are lyrics from the “Major-General's Song” from the 1879 opera The Pirates of Penzance.
Also from "Grim in Love": Malaria and Grim dance the twist in a scene almost exactly like Mia and Vincent's in Pulp Fiction.
Sperg's band Purple Filth from "Battle of the Bands" is likely named after the real metal band Purple Popcorn.
In that same episode, when Sperg tells Grim to play his guitar, to which Grim replies, "Well, I did play that Stairway To Heaven thing in high school."
Billy's line "Play that funky music Grim boy" is a clear reference to the song "Play That Funky Music" by Wild Cherry
Mandy's line, "We accept you. We accept you. One of us." is a shout-out to the lyrics of "Pinhead" by The Ramones, itself a shout-out to the 1932 film Freaks.
A lot of the random lines that Mandy says after the opening titles qualify as these. Another example is when she says, "The call is coming from inside the house." This is a shout out to the famous Wham Line from When a Stranger Calls.
When Billy is asked what weapon he wants for a roleplaying adventure, he shouts "Warhammer!".
Nergal and his Nergalings might be a shout-out to Warhammer 40,000's Chaos God of Decay, Nurgle. Nergal's words to his son about romance sound suspiciously like Nurgle's attitude toward Isha, whom he imprisoned in his lab. Both beings are the namesakes of the Sumerian deity Nurgal.
Billy sees what looks like the shadow of a giant spider on his ceiling. He looks down to see that it is just Bubbles' stuffed octopus on top of his lamp.
One episode has a bunch zombies in a plane which has Air Voorhees written on its side.
At the end of "The Really Odd Couple", Mandy seals Billy in his closet behind a brick wall a la "The Cask of Amontillado". While she's doing so, Billy mumbles something similar to Fortunado's line "For the love of God, Montresor!" behind his gag.
You've got to look pretty carefully to see it, but in some shots of Billy's room throughout the series, you'll notice a figure of Robot Jones.
In "Recipe for Disaster", Mandy ends her big motivational speech to her Person Scout troop with, "Resistance is Futile!"
A possible shout out to the original short in "Skarred For Life" when Billy finds Skarrs "Stabbin-Flabber" he wants it to stab him in the forehead.
The titular character from "Duck!" and its habit of making people think its victims are farting is a nod to a scene from Caddyshack in which Rodney Dangerfield's character says "Who stepped on a duck?" after farting in public.
In "The Crawling Niceness", Billy rummages through Grim's trunk, naming things as he takes them out and throws them to the side.
"...ouija board, eye of etna, devil's triangle, dark crystal, shampoo"
This shout-out would seem like stretch if said crystal didn't look exactly like the one from the movie.
This one's pretty hard to spot due to the lighting, but in the show's intro, the gravestones read "Evil Con Carne", "Time Squad", and "Robot Jones" on them — all short-lived cartoons that were cancelled around the time The Grim Adventures of Billy and Mandy became its own show. See here.
"A Grim Surprise"
In one scene, Billy is wearing a chef's hat and giggles after Grim pokes him in the belly, just like the Pillsbury Dough Boy.
A reference to How the Grinch Stole Christmas! is made when Grim remarks that his heart has grown two sizes larger after he's moved by Billy voicing his appreciation for helping out with the surprise party. We even see an X-ray staff being broken from Grim's swelling heart and a shot of Grim smiling like the Grinch.