The special DVD-only episode of .hack//SIGN, .hack//UNISON, which effectively concluded the original Morganna saga, consisted primarily of various characters from the series and the games gathering for a grand party, which culminates in an all-out dance.
The ends of later arcs in One Piece tend to have these. They ARE pirates, after all. Though there usually is some dancing when there is music involved, their parties revolve more around eating (especially in Luffy's case) and getting drunk, however.
Carl Barks's In Old California...Sort of, in a rather bittersweet manner. Donald Duck and his nephews mighthave gone back in time to Spanish-ruled California and befriended a charming little family. Once they return to the present, they go where the family hacienda was...to find nothing but ruins, since the family, if it really existed, is of course long gone. So they erupt in dance and singing, as a way to honor the family they learned so much from...Just in time for someone to pass by and mumble about these "crazy tourists!"
Film - Animated
Toy Story 2, with Wheezy singing "You Got A Friend In Me"
Toy Story 3 has two separate dance parties in the credits:
One is of the Sunnyside daycare toys dancing; it is implied that once they got rid of Lotso, they could all enjoy the place without fear of reprisal from him. In turn, several of the toys, including former minions of Lotso, were having a big dance party, with "We Belong Together" playing. A happy ending indeed.
The other is Buzz and Jessie dancing to a Spanish version of "You Got A Friend In Me."
Despicable Me, with the girls' just-for-Gru ballet recital turning into a disco party. Vector dances along on the moon.
The first film had the wedding reception held at the end. Everyone danced to the Smash Mouth cover of "I'm a Believer", sung by Donkey. The home video and DVD releases added a Karaoke Dance Party sequence on top of this.
The ending of Shrek 2: dancing to "Livin' La Vida Loca".
Shrek The Third is the sole movie in the franchise that doesn't technically end with a dance party. It gets relegated to the credits where Donkey and Puss do a duet of "Thank You for Letting Me Be Myself."
Get Over It features a dance party led by supporting character Sisqo. He partners with Vitamin C to do a cover of "September." The rest of the cast, including future big stars Kirsten Dunst, Ben Foster, Zoe Saldana, Mila Kunis, and, yes, Carmen Electra get together for a five-minute boogie over the closing credits.
Girl Walks Into A Bar ends with three of the main leads dancing in a bar.
The Great Outdoors ends with Dan Aykroyd and the cast dancing in the hotel's bar to the song "Land of 1,000 Dances."
Harriet the Spy finished with the characters dancing to James Brown's "Get Up Offa That Thing".
Hitch ends with the cast dancing to an upbeat song at a wedding.
The Holiday ends with the two couples and their kids dancing around a Christmas tree.
Not quite at the end, but the climax of Teenage Mutant Ninja Turtles 2: The Secret of the Ooze had our boys in green fighting the Foot Clan in a dance club, then getting up on stage to sing and dance with Vanilla Ice.
Appears in the Scrubs episodes "My Bright Idea", where Carla becomes pregnant, and "My Cold Shower", after Keith proposes to Elliot, as everyone dances at the local bar. Subverted when, after what is otherwise a happy occasion, we see JD taking a cold shower thinking "It should have been me."
The Prisoner: In a defining moment for British television, and one the most notorious pre-Gainax Gainax Ending sequences, almost the entire second half of the final episode is a Dance Party Ending. Really. Then, credits.
Saturday Night Live was known for this trope, especially during the late '80s; when the writers couldn't think of a way to end a sketch, they'd have the characters shout, "Let's Lambada!" Frequently happened on Wayne's World and Sprockets.
Dieter: Now is the time on Sprockets when we dance!
Return to Cranford ends with everyone doing an elegant and stately waltz — which was actually quite a risque dance for the time period. But good God did they have to earn it!!
This trope isn't limited to visual media. Sometimes albums end with songs about dancing!
"Dance, Dance, Dance" finished I'm With You by the Red Hot Chili Peppers, putting a cheery ending to an otherwise dark latter half of the album.
"Stay Young Go Dancing" by Death Cab For Cutie finishes Codes and Keys. This is likely intentional— Ben Gibbard had wanted to write happier music at the time, and even if it doesn't apply to the rest of the album, it certainly applies to this song.
Blue Moves, one of Elton John's darker albums, ends with the exuberant "Bite your Lip (Get Up and Dance)".
The ending to Bananarama's "Cruel Summer" video. Complete with bonfire.
Hank Williams Jr.'s "All My Rowdy Friends Are Coming Over Tonight." Not only did it end with a party (with all of Bocephus' rowdy friends), the party went on (by some accounts) for at least three days after shooting for the video ended.
Many Renaissance Festivals conclude a day at the faire with a Pub Sing or a similar musical finale in which the various musical acts and members of the Village Ensemble lead everyone in song and dance. The festival's monarch (King Henry VIII or Queen Elizabeth I at most Ren faires) usually attends. There's much singing and dancing, and the audience is invited to sing and dance along. Of course, towards the end of the Pub Sing/finale, there's often a soft, sentimental song such as "Wild Mountain Thyme" to let the audience know the merriment for today is nearly over.
Many operas with happy endings have the ensemble do a short dance, especially the 18th-century ones labeled "tragedia con lieto fine" (Mozart's Idomeneo being a good example).
Gilbert and Sullivan, oh yes they do! With laughing song and merry dance!
There is some evidence to suggest that in Shakespeare's day, many plays (including tragedies) ended with all the players getting up and doing a rollicking jig for the audience. The evidence for this includes:
The play within a play (which is a tragedy) in A Midsummer Night's Dream. And after it, in place of the epilogue, Theseus asks them for their Bergomask, which is a rustic country dance.
For bonus points, after they've gone to bed, the fairies show up and have a dance party of their own.
The end of Twelfth Night, with the song "When but I was and a little tiny boy." Some productions make the song a dark epilogue instead.
As mentioned in Film, a lot of musicals end this way.
After curtain call, most productions of Hair end with audience members joining the cast on stage to sing and dance to "Let The Sunshine In," making it a pretty inclusive Dance Party Ending.
"Letting Go" from the 2008 and 2011 runs of Vanities: A New Musical. Ironically, it takes place right after Mary's mother's funeral.
"You Can't Stop The Beat" from Hairspray.
The reprises of "Dancing Queen" and "Mamma Mia" from said show, plus the introduction of "Waterloo!"
Space Channel 5 is a series that is all about dancing. The entirety of both games, start to finish, has the characters (all background characters included) dancing along with the music, since it's a rhythm game. The second game ends with, after just defeating a bad guy whose entire M.O. was getting the entire galaxy to dance, the entire galaxy breaking out into dance in order to lead into the credits. Every character from the game is seen dancing in parade format down a rainbow summoned, presumably, from the power of dance, right through space.
Parodied in Banjo Tooie, when all your allies get together and and have a dance party...before you fight the final boss. They won't let you join until you've finished saving the world. Then once you go back after defeating Gruntilda, the party's already over and they've eaten all the food. So they celebrate instead by playing hackey-sack with Gruntilda's skull.
The original game did this too, after you rescue Tooty... who then berates you after the credits to get off your lazy ass and go finish off Gruntilda.
Super Mario RPG ends with the Mushroom Kingdom throwing a parade while the credits roll.
Given the game's concept, Dance Dance Revolution has done this in its credit rolls a few times. The earliest one was just a black screen with the chosen character dancing, but later versions have featured dance-themed endings; including SuperNOVA 2 (which mainly had some space related imagery interspersed with some shots of the characters), the Hottest Party series (which has usually involved silhouettes of a dancer), X (which had live action scenes of people playing the game itself), and X2.
However, at the same time they've managed to subvert this too; the Japanese console version of Extreme started with an acoustic version of "Graduation" accompanied by the title screens of all the major arcade versions, but then switched to gameplay footage from past versions (which did have characters, a feature dropped from arcade versions on DDRMAX, but backported into the home versions)
The Typing Of The Dead has a bunch of zombies in glass tubes at the end — but if you type out the names in the credits, they bust out and do their best Thriller impression.
'Splosion Man has this as its finale - by 'Splosion Man cosplayers, no less.
The Wheel of Time computer game had this after the end credits. Watching a Myrddraal dance is pretty disturbing, actually.
Bayonetta has a very long dance sequence at the end involving Bayonetta dancing with just about every character in the game.
Jet Force Gemini subverts, parodies, or does something to this trope, seeing as it first ends with a standard award ceremony celebration with fireworks and trumpets and the usual sort... and after a fadeout and a record scratch, Juno is hilariously badly getting down with his bad self in a nightclub the team visited earlier as the credits roll. His teammates, Vela and Lupus, are too embarrassed of him to join.
The Neverhood ends with Hoborg, king of the Neverhood, first making some more people—since he and Klaymen don't really make for a "party" by themselves. Then the party itself happens. "And now it is time... to GOOF OFF!"
Outcast (1999). Complete enough of the game and it's side quests and you get a secret dance credits for the cast. Once the game ends, it shows the percentage you've achieved. Something above 90 % is required.
Syphon Filter 2 has a bonus clip that features all the game's main characters, friend and foe alike, dancing at a disco club. Appropriately enough, the clip is titled, "Congratulations! Party Time!"
Bloodwings: Pumpkinhead's Revenge ends with Pumpkinhead dancing around after you give him the item meant to "quell his soul". Notably, the actor in the suit had to remove the feet in order to do so. It counts since he's the last surviving character. It's also enough to drive you to madness.
The end credits of Transformers: Fall of Cybertron has Jazz and Bumblebee dancing while Soundwave plays a remix of "The Touch".
In My Sims Kingdom, while you certainly haven't completed all the available tasks, the reward for achieving level 5 is your own island, named <player name>'s Island — which looks suspiciously like the Isle of Magic — where you can build without using Mana. Your receipt of this reward is kicked off by a dance party attended by every Sim on each island you've visited by that point, and the credits.
The Citadel DLC for Mass Effect 3, the last piece of content released for that set of characters even though it's before the ending, ends with all the surviving crew members having one of these.
The Simpsons episode "Burns Baby Burns" (featuring the late, great Rodney Dangerfield) lampshades this, as Journey's "Any Way You Want It" plays them out to the credits. The sudden abundance of music and drinks from nowhere is lampshaded.
In "Tales from the Public Domain", after finishing the Hamlet portion of the story, Homer claims that it was made into the film Ghostbusters. Cue the family dancing to the movie's theme song.
You can pretty much take it to the bank that at least one episode in a season will have a dance party ending.
The previous finale, So The Drama and the episode "Queen Bebe" do it too. Both parties are previously established - the first is Kim's junior prom, and resolved the romantic loose end of Kim and Ron hooking up.
The Bugs Bunny short "Fresh Hare" has an ending in which, having been asked by Canadian Mountie Elmer Fudd if he has any last requests before he's executed, Bugs bursts into a rendition of "I Wish I Was in Dixie" and manages to rope the firing squad and Elmer into the act. Unfortunately, it becomes a minstrel show in the process, so since The Nineties the scene is usually cut from television airings.
Galaxy Rangers episode "Marshmallow Trees." Yay, the mutated bugs transformed into high-grade compost, and the problem with the eponymous trees has been solved. Let's break out the fiddles and banjos, folks. (OK, we're dealing with a Space Western...)
Garfield Gets Real used this rather suddenly in the end, with everyone dancing as soon as Garfield and Odie get home.
And another at the end of "Rollercoaster: The Musical!", where pretty much everyone we've ever met gets into the act. Also includes a cameo by Kenny Ortega!
In a slight variation, the Season 3 finale of ReBoot ended with the cast attending a Broadway style musical recapping the events of that season sung to the tune of the Major General Song.
Happens at the end of Tokyo Mater where everyone in Tokyo, Japan actually celebrate Mater's victory over Kabuto by having a dance party in the streets of Japan, with Kabuto being stripped naked as a result of him losing to Mater in a race.
My Little Pony: Friendship Is Magic, "A Canterlot Wedding, Part 2": After all the chaos of the two-part season finale has died down, the episode ends with the real wedding between Shining Armor and Princess Cadance, including the ponies dancing at the reception (organized by Pinkie Pie, of course).
Same with World War One. Even the Germans were happy the war was done. The dance party was short-lived for them though.
Seems to happen frequently in Middle Eastern countries as well.
School proms, of course. Or really, any end-of-the-year dance.
New Year's Eve.
Cameron Diaz tends to do this a lot with all her comedy movies.
The Olympic Games Opening Ceremonies are thoroughly staged and planned, have the solemn Lighting of the Flame, and generally do their best to impress everyone with awe and majesty. The Closing Ceremonies, on the other hand, are basically one big excuse for a party. With, you guessed it, dancing. And rock stars. And the host country good-naturedly spoofing themselves. As far as the "dance party with good-natured self-spoofing" goes, Canada seems to have won that crown in Vancouver.
Whew! We finally finished the entire page... Let's dance!