A subcategory of Romantic Comedy, where the Official Couple were once married, but have divorced each other. Eventually they decide that they want to be married to each other after all. A major staple of the classic Hollywood Screwball Comedy, many examples of this genre owe their existence to The Hays Code, although the trope existed before that. Since the Code prohibited depictions of infidelity, it was necessary to have the characters divorce, then fool around with other people, then get back together.
This last also had the advantage that it made it easier to avoid either one having to be the 'villain' in the previous divorce, and for some, makes them easier to like.
It's practically a given that either the ex-wife or ex-husband has a Disposable FiancÚ.
See also Divorce Is Temporary.
Some endings of Silent Hill 2 ("In Water" and "Rebirth", also possibly "Leave") are somewhat like a horror version of this trope, considering the couple in question didn't seperate through divorce...
Scrubs plays this in reverse with Dr. Cox and Jordan: first they were married, then got divorced. Then they find out that due to a clerical error, they're still married. Due to their extremely screwed-up personalities, this fact causes them no end of torment and so they decide to divorce again to save their relationship.
Twister, the divorce is in progress. Jo never signed the papers, so Bill tracks her down, dragging his current fiancee along. On their first meeting it takes the viewers about two seconds to realize that Bill and Jo are going to get back together.
Curtains has Georgia and Aaron, songwriters and former married couple. They get back together with some suberb acting by some of the other cast members, who wanted the two back together.
Subverted in Mrs. Doubtfire, where Robin Williams' character wants to get back together with his ex-wife (or is at least annoyed to see her with another man), but they never do. Interestingly, Executive Meddling at one point tried to change the ending to play this trope straight only for Robin Williams and Sally Field to convince them otherwise, claiming it would not only be unrealistic, but could potentially cause false hope in children of divorced parents who saw the film. The Happy Ending of the movie is that everyone comes to terms with the divorce and Williams' character at least gets to remain a part of his children's lives.
In O Brother, Where Art Thou?, George Clooney plays a convict who escapes from jail to reconcile with his wife, who divorced him out of shame after his conviction. After all manner of shenanigans, they get back together.
Ocean's Eleven also has George Clooney as a convict, although this time a paroled one, trying to patch things up with his wife, who divorced him out of shame after his conviction. He wins her back while committing even more crime.
Hey Arnold! played with this trope in one episode, with Coach Jack Wittenberg remarrying Tish Wittenberg (helped muchly by Arnold's matchmaking prowess).
It's Complicated comes close to this.
Jude the Obscure is something of a tragedy of remarriage. (It also works the other way round: they both get married to different people, then live together, get divorced from their respective spouses, then separate and get married to their previous spouses again).
Liar Liar, though a little more realistic to be fair, since the scene where Max's parents are interested in each other again is about a year after the main plot has been resolved.
Subverted in The Auteur. Arturo eventually accepts that Fiona won't return to him, because of his "highly addictive personality" and "insanely jealous nature"; but she forgives him his past, and they do manage to become friends again.
The Radioland Murders has this as a major subplot - it's mainly a zany romp through, oddly enough, radio shows and serial killing.
In a The Simpsons episode where Milhouse's parents divorce, Homer and Marge start to question the sanity of their own marriage. Homer begins to feel that the wedding and marriage he provided for Marge was less than she deserved, so he gets divorce papers...and then throws a special wedding in the house, so that they could say their marriage was great from the start. Several seasons later, it turns out that Rev. Lovejoy, who officiated, was not legally (or religiously) able to do so at the time because of a technicality, so Marge and Homer end up having another "real" wedding, and Marge becomes a Bridezilla.
Milhouse's parents remain divorced for many seasons, but eventually rekindle their relationship and get back together.