Too many to count in The Sandman (1989), but the most memorable is the The Sandman: Endless Nights story, "The Heart of A Star" — Superman's ultimate origin is revealed to be a conversation between Krypton's sun and the first personification of Despair on the nature of beauty.
Which is a brilliant Genius Bonus by Gaiman. The idea is that Superman only survived so he could feel the despair of being the last Kryptonian - but of course, Despair always fails.
This story also features a character called Killalla of the Glow, who is implied to be one of the first aliens to master the energy later marshaled by the Green Lantern Corps.
Heck, she's as-good-as-stated to be one of the Guardians, before the female Oans went their separate way.
It's never directly stated, but there are enough hints to figure out that Matthew the raven was Matthew Cable, from Alan Moore's series Swamp Thing, before he died. Gaiman owes a lot to Moore in getting his career started; there are several nods in Sandman to Moore's work in the DCU.
Writers and artists often use scenes set in the Batcave's trophy room to give a Continuity Nod to previous Batman stories. Traditionally included are the giant penny from a 1947 fight with The Penny Plunderer, a T-Rex from a 1946 adventure on Dinosaur Island, and a memorial to Jason Todd, first seen in The Dark Knight Returns.
Also, Batman once investigated murders at a movie set, where the movie being shot was a WWI flying action/drama. The misunderstood Villain Protagonist of said film was Hans von Hammer, in our world the main character of DC Comics' Enemy Ace series.
In Supergirl (Rebirth) #1, Director Chase mentions that Supergirl's space pod's landing place was Siberia (which happened way back in Supergirl Vol 6 #1).
In Supergirl story Young Love Pre-Crisis Supergirl remembers several moments of her childhood:
Arriving on Earth and meeting Superman (Action Comics #252)
Dick Malverne taking a picture of Supergirl by chance (Action Comics #256)
Linda accompanying her parents on a beach-picnic (Action Comics #282)
Linda and Dick going skating (Action Comics #283)
Their reunion in Supergirl Vol 2 #23 when they are in their twenties.
Occurs regularly in Astro City, as befitting a series with a single writer. Most Continuity Nods appear as sidelong references to other characters and events in the chronological past/present, even if the subject hasn't had a published appearance yet.
Samaritan briefly mutters "3.2" when he arrives to visit Steeljack in "The Tarnished Angel." This is a reference to Samaritan's Day in the Life story, "In Dreams," where it's shown he keeps track of how many seconds he spends flying from one scene to another.
In "Show 'Em All", as Jack-In-The-Box dodges The Junkman's aerosol bombs, he casually mentions having "recent experience" in dodging mid-air explosions. This refers to a single panel from the earlier "Confession" story arc, where Jack-In-The-Box eludes capture from a missile-firing helicopter.
Also from "Confession", Brian begins his super-hero career by working as a busboy in Bruiser's Bar & Grill, run by retired Golden Age hero The Black Badge. Both the Black Badge and Bruisers' play small but pivotal roles in the later "Dark Ages" story arc.
Kurt Busiekloves to put Continuity Nods in all of his works, helped by his encyclopedic knowledge of comic book history.
In Issue #2 of Red Hood and the Outlaws, Red Hood gets an air hostess' number. They met in Batman #426, 20 years ago. She still remembers his drink order, although in modern continuity, they probably met 2-3 years ago. And he mentions 'A Death in the Family', although that is a Shout-Out. Her name is Isabel Ardila, by the way.
In issue #6, it's revealed that Red Hood's costume is actually one of Nightwing's old costumes. Though in-universe information is lacking thanks to the recent reboot, it seems to be based on Nightwing's Renegade costume.
The Night of the Owls tie-in nods to the revelation in Batman that Haley's Circus (of which Nightwing was a performer) was a recruitment service for the Court of Owls. It may also be a reference to Red Hood's pre-Crisis history as a circus performer.
The Death of the Family tie-in has the Joker forcing Jason to relive the circumstances of his death in the original A Death in the Family arc.
Also in reference to Death of the Family, Penguin is seen talking to Bane about The Joker and paying him back for what happened.
Amethyst, Princess of Gemworld: When Amethyst learns how to form shields with her magic she compares it to Wonder Woman's bracelets. Makes sense with how there's a very prominent poster of the Amazon Princess on her bedroom wall at home.
A Death in the Family was possibly the first 'mainline' comic to acknowledge the events of The Killing Joke, noting the superhero community and especially Jason are furious with the Joker for what he did to Barbara. Later, the wheelchair-bound Barbara is seen attending Jason's funeral.
