In one episode, Jonathan (played by John Schneider) is driving his pickup truck while The Dukes of Hazzard theme song plays on the radio. In another Dukes of Hazzard allusion, when a State Senator played by Tom Wopat shows up, we learn that he and Jonathan are best friends. John Schneider said, according to the DVD commentary on Season 1, that he was in an airport around the time the show premiered, and it was the first time in 20 years he was recognized for a role other than being a Duke boy.
"Precipice" has the new sheriff telling Jonathan to his face that "those Good Ol' Boy days are over".
The episode with Tom Wopat has them bringing up that he was "under the bleachers with the sheriff's daughters", which is hilarious as the sheriff on Dukes of Hazzard was the Ineffectual Sympathetic Villain. And Lex is the one who says "good ol' boy" this time.
When Margot Kidder, the Lois Lane actress from Chris Reeve's Superman movies, shows up as the assistant to Reeve's character Dr. Swann, she implies that she and Dr. Swann were/are attracted to each other. When questioned as to whether they had ever been in a relationship together, Kidder's character wistfully replies "In another lifetime."
When Dean Cain, Lois and Clark's Clark Kent actor, shows up, we briefly see a handkerchief with his character's initials: "C.K."
James Marsters, famous for playing the vampire Spike on Buffy the Vampire Slayer, plays a professor who denies the existence of vampires. The main villain of the episode is a girl named Buffy who has been turned into a vampire-like being. Though the Buffy name is an in-universe reference, as Chloe says she's changed the names.
Of perhaps the most ironic kind. The producers of Smallville wanted the character of Bruce Wayne to appear. The owner of the current Batman film franchise refused. The irony is DC comics owns both characters and in the comic books both Superman and Batman crossover all of the time, and are, in fact, best friends. Batman finally appears in the Season 11 comics' second story arc.
Stephanie Brown was to appear as Batman's sidekick, Nightwing. But, of course, anyone familiar with DC knows that the higher ups hate her with a passion, so Barbara Gordon was used instead.
Lana: In the earliest seasons she was initially known as the Pink Princess because of her early wardrobe (and the fact that by season 3 the pinkness was arguably becoming a Running Gag until her wardrobe changed in Season 4). Super Kryptonite Woman or The Amazing Kryptonite Woman, after becoming walking kryptonite. Also known as the "squirrel".
Pete: Product Placement Pete, as his little bit of screentime was usually used to place products such as music from Lifehouse, the "Talon Mix" CD, and Stride Gum.
Lois: EDLois (short for Erica Durance).
The episode "Doomsday" is often nicknamed "Failsday" for its rather anticlimatic ending.
Friday Night Death Slot: Was moved to this in its second-to-last season, but remained one of The CW's top shows. Its second season in this slot was its last, but that was less due to time slot and more due to factors such as various contracts expiring and a general feeling that it was time to finally end it.
Michael Shanks as Carter Hall/Hawkman. Oddly enough, this means that that he's once again playing a character with an interest in Ancient Egypt. Also, both characters have also experienced a number of deaths that didn't quite stick. Plus, both characters have similar sounding names for the woman they love (Sha're/Shayera).
Minor examples, but Jodi is Giselle. And a few people may recognize voice actor Andrew Francis as the student who tried to run Chloe off the road for exposing his mother's old life (and that she caused people to die) in Season 3's "Truth." This show gets quite a bit of this, probably because... well, it's Superman we're talking about.
Cassidy Freeman, who plays Tess Mercer, is the sister of voice actor Crispin Freeman (who has coincidentally happened to voice Superman)
And as a possible Casting Gag, Tess was shown to be someone against Clark and working for Lex Luthor before siding with Clark. note Crispin voiced Superman in a JL game and was the voice for Red Arrow in Young Justice, therefore both Freemans have been allied with the Big Blue Boy Scout.
Averting Christopher Reeve's intractable marriage to the iconic role was among the reasons Tom Welling had insisted on "no tights, no flights" in his contract. But if the role doesn't stick to you after 10 seasons, it's not going to happen.
What Could Have Been: Allison Mack toyed with the idea of auditioning for Lana Lang before going for Chloe Sullivan. Incidentally, Allison is a green-eyed Caucasian just like traditional Lana while Kristen Kreuk/Smallville Lana is part-Chinese with dark eyes. While Allison is a natural blonde, the actress of Maxima also dyed her hair from gold to red for the role.
The writers planned to bring Bruce Wayne into the show at one point as a guest role, but this was shot down because of the then-upcoming Batman film. His role and storyline was apparently converted into Adam Knight instead.