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), Nois ("No is", as in, "is not Lois").
The episode Doomsday is often nicknamed "Failsday" for its rather anticlimatic ending.
Annette O'Toole (Lana Lang in Superman III) as Clark's mom, John Schneider as his dad, Terence Stamp's inimitable voice as Jor-El (and a cameo as Zod), Michael McKean as Perry White, Rutger Hauer as (pre-surgery) Morgan Edge, James Marsters (who happened to voice Lex Luthor once) as Brainiac, Sam Witwer as Davis Bloome, Alessandro Juliani (L) as Dr. Emil Hamilton. Jane Seymour, Helen Slater, Dean Cain, Margot Kidder and Christopher Reeve all have small roles.
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 as a murderer in Season 3's "Truth."
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 is currently voicing Red Arrow in Young Justice, therefore both Freemans have been allied with the Big Blue Boy Scout.
Of course, he's far better known for playing a guy named Dean. In fact, that's the role he actually left Smallville for.
Meghan Ory, now well-known for her role as Red Riding Hood on Once Upon a Time, played a cheerleader who is almost killed in a season three episode.
What Could Have Been: Allison Mack toyed with the idea of auctioning for Lana Lang before going for Chloe Sullivan. Had she been cast for the former, we may have been able to enjoy eight seasons of Clark pining after someone who actually has chemistry with him and not spend the time banging off walls and/or screaming "make it stop".
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.