Doctor Who had two films starring Peter Cushing as 'Dr. Who' (his real name) which took place outside the series' continuity. "The Five Doctors", a 90-minute special to celebrate the 20th anniversary, was basically a Movie. A canon film was the 1996 Paul McGann TV movie, which was an attempt to get the show back on track after its original 1989 cancelling. The 50th anniversary special is being billed as a movie, and showing in theaters along with the global simulcast.
Firefly spawned a theatrical film, Serenity, which tied up most (but not all) of the dangling threads from the short-lived series. Universally known to fandom as the Big Damn Movie.Fan Fiction even uses this to help identify the time frame: Post Show Pre BDM, Post BDM, No BDM. There are some that are Post BDM + AU.
...and In America! Where it appeared in some theaters ... in Sensurround!
Mystery Science Theater 3000 's movie was actually shorter than any episode of the TV series - having apparently had a few of the 'host segments' cut right out just prior to release - and featured slightly dumbed-down riffing (i.e. fewer obscure references, more crude language) as well as an actually tolerable movie (This Island Earth, considered by many to be a sci-fi classic) due to Executive Meddling. This is not to say the movie isn't still pretty funny.
This Island Earth was itself edited down significantly for use in the MST3K movie which- even including the new footage- still ran almost 15 minutes shorter than the original film.
Star Trek and Star Trek: The Next Generation have had 10 films between them — six of the former and four of the latter. In a more conventional way of speaking, however, the first one (Star Trek: The Motion Picture) was theThe Movie for TOS and the seventh and first unnumbered one (Star Trek: Generations) for TNG—the sequels to them essentially icing on the The Movie cake, allowing their respective shows to basically continue well past their final TV episodes and to engage in the Grand Finales they didn't have during their original run. Then again, if you have not heard about the "even-odd rule"... basically, there is a strange pattern in which the even-numbered films are (in general) better than the odd-numbered films. The result is that the second TOS film, The Wrath of Khan, is usually considered their best, and the second TNG film (movie #8), First Contact, is also considered the quintessential TNG film, distilling all that was best in their respective series. Still, their first movie outings still had that "The Movie" effect, where everything was a shiny new movie set focusing more on "wow, we're actually in a theatrical film now". Of course, of the four TNG films - movies #7 through #10 - #7 and #9 fell victim to the odd-numbered rule (though #9 is more like a pleasant stand-alone, the cast officially thinks it's boring), and #10 (Nemesis) officially broke the "even numbered Trek films are great" rule (the cast openly stated that "it sucked"), First Contact was the only TNG film universally accepted as a great entry.
Rescue From Gilligan's Island had the cast finally escape the island, struggle to reintegrate into normal life, and finally become shipwrecked on the island once again during a reunion cruise.
The Castaways on Gilligan's Island saw them escape yet again, but return to open a holiday resort. This was intended to be the pilot for a Spin-Off that never eventuated.
The Harlem Globetrotters on Gilligan's Island saw, you guessed it, The Harlem Globetrotters crash landing on the island before helping to thwart the schemes of a Mad Scientist who wants the island for its rich energy supply. The issue is, naturally, settled in a basketball match between The Globetrotters and the Mad Scientist's robotic team.
Noah's Arc: Jumping the Broom, the movie about Wade and Noah's wedding (and the craziness that precedes it).
After already being a spinoff of the original Stargate movie, once Stargate SG-1 was canceled, the major plot of the ninth and tenth seasons was brought to a climax in the movie Stargate: The Ark of Truth. This was then followed by a sequel called Stargate Continuum. Further movies were apparently planned, including another SG-1 movie, an SGA movie, and an SGU movie, but these have since devolved into Development Hell if not cancelled altogether.
The unusual thing about Stargate fandom is that the film referred to as "The Movie" is actually notThe Movie in the sense of this trope; it was the first film that everything else is based off of (essentially the opposite of The Movie). The reason for this is simply that it's the only part of the entire franchise that doesn't have a subtitle, so there's pretty much nothing else to call it. And then it gets even more complicated because the cast and creators of the film don't consider the series canon, and have frequently remarked on their desire to complete the film series as they intended and ignoring the spinoff entirely, while the series in turn diverges from the movie in a number of areas as well.
So while other shows might have The Series followed by the Big Damn Movie, Stargate has The Movie followed by the Big Damn Series. (This is considering that in the first movie they only saved one planet, whereas afterwards they go on to save multiple galaxies several times over and uncover the Secrets of the Universe).
Numerous British Sit Coms of the 1970s had movie spinoffs, featuring the original cast and (usually) writers, but filmed on different sets and locations. These films typically featured the cast going on a bus trip, or a cheap foreign holiday, and generally had poor reviews. Examples include:
It's worth recording Nick Hornby's reaction to these things, when he was watching them as a kid. 'For me, growing up, the cinemas wasn't a place of magic and wonder. It was where old sitcoms went to die.'
Thunderbirds: The series was followed by two theatrical movies, which were unexpected box-office flops (in fact, the second film was greenlighted despite the first one being a flop as they thought it was a fluke!).
In one of the largest dichotomies in this trope, the otherwise-cheery and humorous Sesame Street had Follow That Bird, a full-length Tear Jerker of a film that delves into much darker territory of family, loss, and self-identity than anything that ever showed up in the normal show. This was followed over a decade later by Elmo in Grouchland.
Family Ties Vacation sends the cast to Merrie Olde England after Alex wins a summer scholarship to Oxford.
The Facts of Life had more than one of these: The Facts of Life Goes to Paris sends Mrs. Garrett and the girls to France, while The Facts of Life Down Under sends them to Australia. There was also a reunion movie about a decade after the show ended.
Moving into the '90s, Sabrina the Teenage Witch had two during the run of the live-action show: Sabrina Goes To Rome and Sabrina Down Under. Unlike most movies of this ilk, the majority of the regular cast weren't involved (in both cases only Sabrina and Salem got to make the trip).
Leave It to Beaver had a reunion movie (Still the Beaver) which led to a new series (titled, creatively enough, The New Leave It To Beaver) in the '80s. It also had a very forgettable theatrical version, with a whole new cast and set in more modern times, in 1997.
The Brady Bunch had two reunion movies The Brady Girls Get Married, A Very Brady Christmas. Each of these spawned a short-lived series (The Brady Brides, The Bradys).
Columbo started as an ABC Mystery Movie of the Week, then spawned a TV series, then spawned even more TV Movies!
Sex and the City took 4 years to get the movie out. Originally planned for the year after, but Kim Cattrall wanted more money for the work. (and who can blame her?) So it dragged on for a while until Kim got a deal.
Get Smart had planned to make a full-length movie, which instead became the three-episode story arc "A Man Called Smart". Way after the series ended, there was a bad movie, a made-for-tv movie, and another 2008 movie.
Power Rangers got two movies during the height of their popularity. The first is a non-canon film based on Mighty Morphin' Power Rangers: The Movie. It introduced early the ninja powers they would be using for Season 3 (which later formed their own story arc on how they got those powers). The second was used to introduce the Power Rangers Turbo series. This one was part of the TV show's continuity.
On the Japanese end of things, every year (since the early 2000s) there's a Super Sentai/Kamen Rider double-feature (aka Super Hero Time: The Movie (insert year here)), with the Sentai movie getting about half the screen time the Rider movies get. (In contrast to America, where toku shows can be divided into the categories of "Power Rangers" and "everyone else", Kamen Rider is much bigger than Super Sentai in Japan.)
Super Sentai also has a team-up with the current team and its predecessor every January. Kamen Rider is getting in on the action with that lately. And now, Rider/Sentai teamups are also going to be an every year thing. So by now, you've got a Super Hero Time double-feature, a sentai 'versus' movie, a Kamen Rider 'Movie Wars' movie, and a KR/SS movie every year. Also, if a series is popular enough, it'll get a movie or two after its run finishes, though usually DVD only. Japan loves its Toku.
Monty Python refilmed a bunch of their TV sketches and released it in 1972 as And Now For Something Completely Different. It became must-see viewing on the midnight movie circuit.
Barney & Friends had Barney's Great Adventure, a movie where Barney and the kids have to find a rainbow egg that has a creature in it that can grant any wish.
There was going to be a Teletubbies movie, but there was a dispute between the American distributor of the Teletubbies and the original creator of the Teletubbies, Anne Wood. This resulted in the Oogieloves.
Little House on the Prarie had quite a few TV-movies after the series' run which gave the show closure, the most notable movie being when the town of Walnut Grove is destroyed.
The Peter Gunn movie Gunn (1967) had Craig Stevens once again as the title character.
Tracker had a movie of sorts that was really a couple of episodes edited together and released as Alien Tracker. The poor quality and fan refusal to buy it is probably what made Lion's Gate think DVDs wouldn't sell, and no season set was ever released.