In the episode, "Let's Try That Again," Madison's step-mom is played by Michelle Krusiec, who is probably better known for her portrayal of Helen Chin on Weeds as the tennis coach Dean Hodes has an affair with in "You Can't Miss the Bear"
Probably the biggest one is Molly Ringwald as Amy and Ashley's mom as most audiences are used to seeing Ringwald as a teenager and not the mother of one. Also has to do with the show's superficial view of teenage pregnancy in sharp contrast to Ringwald's role in For Keeps.
Mimi is the manager at the butcher shop where Ben and Ricky work.
Blossom was the high school counselor in season two and the first half of season three.
During the latter half of season three and season four, the role of high school counselor is taken over by Lucy Camden
Ruthie Camden has a few appearences as Zoe, one of Ricky's casual sex partners.
Renee Olstead(Madison) and Camille Winbush(Lauren) are much more well-known for playing snarky daughters on Still Standing and The Bernie Mac Show respectively. Somewhere in between, they forgot how to act.
Elvin Tibideaux is the court-appointed mediator who handled John's custody case.
It's a little jarring when Grant's very amiable mom is introduced, since she's better recognized as Ax-Crazy Amanda Young from Saw
Apparently Winston gave up his life of ghost hunting to become Lauren's dad and Ricky's therapist.
Jo ends up as one of Ben's recurring friends in the 1st episode of the 2nd half of Season 4.
Money, Dear Boy: Poor Molly Ringwald. In the outside world, show creator Brenda Hampton has said in regards to the many negative reviews that she'd rather the show get good ratings and bad reviews than bad ratings and good reviews.
No Export for You: If you live in the Philippines, and you love this show so much, don't expect it to come there. The Catholic church leaders will just hound this show to death before it can air there. Fortunately this was averted, limiting the show's airing on local cable.
Reality Subtext: Molly Ringwald's pregnancy was written into the storyline.
Saved from Development Hell: The show was shopped around to different networks for about ten years before getting picked up by ABC Family.