Angel: The series makes extensive use of this trope, often to detail events in Angel's past and sometimes other characters too. Many episodes have at least a few minutes of flashbacks and many episodes will feature extensive use of flashbacks that alternate with events of the present time that are somehow connected to what the flashback is detailing. Examples of episodes include "Are You Now or Have You Ever Been", "The Prodigal", "Five by Five", "Darla", "Lullaby", "The Cautionary Tale of Numero Cinco", "Destiny", "Why We Fight", and "The Girl in Question".
Arrested Development: Subverted Trope: in the first season finale, two lines that seem to provoke flashbacks ("Your father promised [the company] to me on the day he went to prison.", "We've had some great times.") are followed by blank screens captioned "Footage Not Found."
Arrow: Every episode divides its time between Oliver Queen's activities in the present as a vigilante and the events in his past that led to his becoming one.
Being Human: Has a beautifully simple one - as the vampire Mitchell walks down a street, internally monologuing, his hairstyle and clothes change to reflect the fashions of every decade since his turning. Being Human has lots of flashbacks, especially in series two, where every single episode begins with a flashback.
Buffy the Vampire Slayer: Used flashbacks frequently to establish the backstories of characters like Angel, Drusilla, and Spike.
Charmed: The nature of Phoebe's power of premonition is that she sees visions of what might happen, and sometimes what had already happened. Then there are spells that have been cast by others to see scenes from both the past and the future.
Doctor Who: While the new series does not use flashbacks often, it has received some stick for using them to flash back to extremely obvious, memorable scenes from previous episodes. This is to help children understand what's going on as - despite its morbidly high body count - the show is aimed at families.
Farscape: The episode "The Ugly Truth" has each of the crew members flashing back to the same set of events, with each giving a different twist on what happened — to the frustration of the captors trying to interrogate them. Obviously these aliens aren't familiar with the concept of The Rashomon.
Firefly: The episode "Out of Gas" which jumps between three time periods: the current time frame (the actions of a dying Mal), a short time in the past (the events that led to Mal's current situation) and a more distant past (how Mal's crew was recruited, including the ship itself).
Almost every episode has a flashback as a part of the case. Most of them are from Nick, the show's main character, but some come from others. The notable exception is the episode 'Games Vampires Play,' in which the spots usually taken up by flashback are used to show Nick playing a virtual reality game.
There are many jokes amongst fans over the fact that Nick would frequently have his flashbacks while driving or mid-conversation. This was lampshaded once or twice by other cars beeping at him at stoplights and characters noticing every once in a while and staring at him.
Friends: with the prom video and Thanksgivings past.
The Golden Girls: Did several episodes each featuring multiple flashbacks on a common theme. It usually felt like the writers had ideas for gags which were not enough for a whole episode, and was often easily mistaken for a Clip Show. There was also an episode showing how the girls met.
Highlander: Virtually every episode has an extensive flashback; since the series protagonist is four centuries old, there's plenty of available plotlines to choose from. Usually, the flashback shows the hero's first meeting with the guest Immortal of the week.
Homeland: Used heavily through the show to explore what happened to Brody during his time in captivity and tease whether his Heel-Face Turn actually occurred or not.
How I Met Your Mother: Because this show, in essence, has every episode as a flashback, along with its quick editing, it's hard to tell where flashbacks end and begin, or if a Flash Forward (such as Barney's brother's wedding) really counts as a flash forward or if the rest of the episode is a flashback compared to the flash forward. No one seems to mind, though, because everyone can still follow the storyline.
The IT Crowd: Does this a couple of times in one episode regarding how one character became a Goth and subsequently lost his high position and was forced to work in a room in the basement. Contains a lot of Flashback Stares.
It's Garry Shandling's Show: Has a flashback booth (labeled "It's Garry Shandling's Flashback Booth") that Garry enters to start a flashback.
Every episode has short flashbacks, usually at the end, when some hidden scene is revealed that explains how the Leverage team outwitted their opponent-of-the-week. The pilot also had flashbacks describing each team member. There was also an episode that was done in Rashomon-style flashbacks.
Features flashbacks extensively from the pilot onwards. The show begins with a plane crash that strands a group of characters on an island. Flashbacks are used to show how the different characters ended up being on the plane in the first place. Early episodes were often "themed" around a single character, with a present conflict on the island being illuminated as their backstory showed what kind of person they were before the crash. After several seasons the backstories of most of the main characters had not only been covered very thoroughly, but some characters' lives were shown through flashback to have been connected even before they got on the plane together. At the end of season 3, the writers subverted the viewers' familiarity with the use of constant flashbacks when the two-part season ender featured a lengthy flashback that appeared at first to show a bearded, alcoholic Jack dealing with the recent death of his father, an event previously established as having happened just before he boarded the doomed plane. While his behaviour and the reaction of people around him in the flashbacks seemed to fit the timeline, they ended with Jack meeting Kate, another island survivor. The flashback was in fact a flash-forward to a time when both characters had finally managed to get off the island, and when viewed again Jack's behavior and his treatment by other people must be re-interpreted in the context of him becoming famous after returning home. This set up the fourth season to feature flash-forwards almost exclusively instead of flashbacks, though episodes two, six, eight and eleven still featured them.
Season 5 started with a series of episodes that either contained no flashbacks or only had brief flashbacks at the start of the episode. After that, we had full flashbacks to how Sawyer joined DHARMA in 1974 and how Locke died. Then, characters in 1977 began to have flashbacks to their past-which happened to be 2007, making it so that we have flashbacks to events that chronologically haven't happened yet.
Mystery Science Theater 3000: Parodied in the episode featuring Boggy Creek 2: The Legend Continues: During a host segment, Crow, Tom Servo, and Mike attempt to explain a fight that occurred minutes ago via three flashbacks, each blurrier than the last, with none of them adding any new information. Mike then realizes that he was looking for his contacts, and Servo promises his next flashback will have a car chase and explosions.
MythQuest: Used in the episode "Blodeuwedd", when characters give testimony about another character's death.
New Amsterdam: The short-lived show has John do flashbacks occasionally, starting with the pilot, where he remembers how he was mortally wounded defending a Magical Native American woman, who repaid him by making him immortal until he found his soulmate. Oftentimes, he remembers past lovers (each time, he thought she was "the one"), children, and dogs.
The Pretender: Hardly has an episode without flashbacks, generally in some way related to whatever is going on in the present.
Psych: Begins every single episode with a flashback to Shawn's childhood, usually vaguely related to the plot in some way.
Revolution: Close to the level of Once an Episode. "Pilot" had a flashback on the Matheson family just before the blackout. "Chained Heat" had a flashback focusing on Ben retrieving his work and Rachel Matheson handling the Wiry Stranger. "No Quarter" had a flashback focusing on Miles Matheson, Sebastian Monroe, and Jeremy Baker. "The Plague Dogs" had a flashback of Maggie being taken in by the Mathesons, as well as a flashback of Rachel putting herself in the custody of Miles. "Soul Train" had a flashback focusing on Tom Neville and his family. "Sex and Drugs" had a flashback focusing on Aaron Pittman and his wife Priscilla. "The Children's Crusade" had a flashback focusing on Ben, Rachel and Randall Flynn. "Ties That Bind" had a flashback focusing on Nora Clayton and her sister Mia. "Kashmir" used hallucinations instead of flashbacks. "Nobody's Fault But Mine" had a flashback focusing the relationship between Miles and Monroe. "The Stand" had a flashback of Ben and Rachel putting Danny through an experimental procedure. "Ghosts" had a flashback focusing on Randall Flynn and the loss of his son. "The Song Remains the Same" had absolutely no flashbacks. "The Night the Lights Went Out in Georgia" had a flashback focusing on the relationship between Miles and his former protege Alec Penner. "Home" had a flashback focusing on the relationship between Miles, Monroe, and Emma. "The Love Boat" had absolutely no flashbacks. "The Longest Day" had a flashback expanding on the reasons Rachel turned herself in to the custody of Miles. "Clue" had absolutely no flashbacks. "Children of Men" had a flashback focusing on the relationship between Ben and Rachel before and after the blackout. "The Dark Tower" had a flashback revealing the circumstances that led to Miles trying to assassinate Monroe.
Terminator: The Sarah Connor Chronicles: Has a flash-forward that technically, doesn't exist, as the character who envisions it (Derek Reese), came back in time and killed the inventor of the device that will eventually become Skynet. Which means, if he did that, then the person would cease to exist, and...oh my god, I've gone cross-eyed!
And another starts at the end of the story, does a How We Got Here flashback to the beginning, proceeds to use several more flashbacks as events of the mystery are unraveled, and when it reaches the end again, it turns out it was All Just a Dream. Surreal, to say the least.