Hayley from American Dad!. Her default outfit shows her midriff (and her navel ring).
In the animated version of Angel's Friends Raf, Urie, Sweet and Miki have exposed this.
In one episode/short/cartoon in Animaniacs called "Belly Button Blues" Katie Ka-Boom wears a midriff bearing shirt that shows off her belly button. Her family all notice it and have different reactions to it, especially her little brother who constantly keeps singing, "I see Katie's belly button! I see Katie's belly button!". It annoys her so much, especially when her DAD mentions her belly button. She becomes enraged and turns to a green monster with an even bigger and MORE hideous belly button!
Mookie from Atomic Puppet, in both his superhero and civilian outfits due to him being rather fat and his shirts clearly being one size too small.
Avatar: The Last Airbender: Ty Lee full time, Mai and Azula in the beach episode, Katara and Suki when trying to blend in with Fire Nation clothes, and most female Fire Nation guards. Player!Ty Lee wears this in "The Ember Island Players" play The Boy in the Iceberg, though she's noticeably heavier set.
Joan of Arc in Clone High wears a short black t-shirt that exposes her navel.
Yumi Ishiyama and Sissi Delmas of Code Lyoko, in both variants of their usual outfits, their sport gears and some party clothes too. In general, crop tops are popular with the female students at Kadic. This trope is even commented on by Tamiya in episode 5, "Big Bug".
The guys aren't totally immune either, as Odd always wears a crop top. Of course, he wears another shirt underneath (except when the animators don't color it properly).
Grownboy from The Crumpets always exposes his midriff thanks to his open vest.
Ms. McBrisk in few of her outfits.
Granny, in one shot in "Ransoming Dad".
Ditzy in her primitive-style clothes in "Prehistoric Crumpets".
Many characters including Ditzy when they're bloated in "Lil Wrinkly One".
Amy Wong is quite committed to this trope. Her default outfit is a pink sweatpants and crop top combination, but almost every non-formal outfit she wears exposes everything between her waist and ribcage. When it's cold out she wears a belly parka.
Leela does this in almost every outfit she wears, the main exception being her default one that she wears 90% of the time. Her other outfits usually feature small cut-out windows around her belly button.
In "Crimes of the Hot", Professor Farnsworth wears a see-through bellyshirt for a spot of Fan Disservice.
In "Three Hundred Big Boys", Farnsworth spends his tax rebate on a stem cell treatment to make him look younger and more attractive. He starts dating an attractive punk girl who wears a tank top that shows off her navel ring. Takes a sharp turn into Fan Disservice once Farnsworth's stem cells wear off and his true age is revealed, prompting the girl to reveal that she also spent her tax rebate on a cosmetic procedure to look more attractive. She removes her navel ring, and instantly becomes extremely fat.
Hyena in Gargoyles ended up with a permanently bare midriff after she became a cyborg. Demona and (in the comics) Angela also tend to go around bare-midriffed. Also Queen Titania had an extremely bared midriff, complete with navel gem. Eliza in her corrupt cop look and her stint as a temporary Gargoyle.
Pizzazz and Roxy from Jem dress this way in a few episodes. It's meant to emphasise their "bad girl" qualities compared to Stormer or the protagonists, though occasionally Hologram members bare their midriffs (far less than Misfits though). This is also Minx's default way of dressing.
The Jetsons: Judy Jetson's default outfit exposes her midriff, although she has no navel drawn in.
Kim did this all throughout her first season, but by the last, all of her outfits had changed to avoid this.
Her dad also comments on it in the episode where she gets her new mission outfit (her classic mission outfit being the final piece of midriff-baring clothing to go), saying "It's been my dream to see you save the world without baring your midriff." before revealing a large, cumbersome suit of Powered Armor for her to try.
Mike from Mike, Lu & Og wears a red tank top that leaves her belly button exposed.
A lot of characters on My Dad the Rock Star show their midriffs. Crystal and Serenity Zilla are probably the most obvious examples, but Rock and Skunk also have visible navels. Downplayed with Alyssa, who only has a slight sliver of her stomach visible.
Main universe Candace often does this when going on a date with Jeremy, changing out of her usual Limited Wardrobe into a pair of jeans and a white cropped T-shirt.
We also see Doof in a croptop in one episode, though this is more like Fan Disservice. Perry is justifiably horrified at the sight.
Stacy has several outfits like this during the "Disco Miniature Golfing Queen" number from "Put that Putter Away".
Isabella, too, has an outfit that shows off her midriff during the same number from "Put that Putter Away", as well as a Hawaiian Hula outfit, mistaken for a "belly dancer" outfit, in "What Do it Do?".
So did Fireside Girl Adyson in "Put That Putter Away".
The background dancers in "Squirrels in My Pants"
A lot of women around the Tri-State area tend to show off their belly buttons (even mothers with kids).
Debbie Turnbull and the PE Teacher in Robotboy. The former wears a pink top that exposes her stomach and the latter wears a two piece outfit.
Just about every single princess in Sea Princesses uses this, and some of the princes as well.
Isabela from Seis Manos wears a shirt that doesn't cover her navel.
Male example: Bow from She-Ra and the Princesses of Power. In "Princess Prom" he even rips the midsection out of a rather fancy suit to fit his style. Mermista also has a little bit going on, but since Bow is one of the main characters, his is on display more often.
Brandine and Shauna from The Simpsons. Also, Barney and the Comic Book Guy, mostly because of how fat they are.
Star Wars: The Clone Wars has Ahsoka, which while not quite as abnormal as it might be for a Jedi considering what's worn by Aayla Secura, still stands out, especially when you consider her age. Her later outfits did away with this.
Gems with their gemstone embedded in their abdomen generally wear clothes that leave it visible in one way or another:
Steven is a downplayed version, as his untucked shirt just barely reaches his waist, but is often lifted enough to show the gem he has in place of a navel. All of Steven's fusion are larger than him, but wear the same shirt, leaving Steven's gem pretty much constantly on display.
Steven's mother Rose has the same gem in the same place, which is visible through a star-sharpedNavel Window in her dress. Her fusion with Pearl, Rainbow Quartz, has a similar cutoff, but it's half-star, half-diamond.
One member of Yellow Diamond's Ruby Squad shows her belly-mounted gem with a triangular cutout based on her waist, which is why Steven nicknames her "Navy". It goes up so far it would show underboobsif she had any.
Pink Diamond's outfit leaves the entire lower front of her torso exposed. The pentagonal border of this opening is aligned to the pentagonal profile of her gem in its center.
White Pearl has a gap between the upper and lower parts of her outfit, with the vertical boundaries forming a diamond shape around her gem.
Lapis Lazuli's halter top has a very open back, as that's where her gem is, but it's also short enough to show off her midriff, revealing that she's Belly Buttonless (as are all Gems, apparently).
On Total Drama, almost all the girls have their midriffs exposed, with some only having a slight sliver without an exposed navel (like Bridgette and Gwen) and others showing off a lot more skin (like Heather and Izzy). Courtney and Dakota's regular outfits don't show midriff, but it is shown on some of their alternate outfits (sleepwear for both, swimwear for Courtney, and an intern outfit for Dakota). Lampshaded when Leshawna and Heather are up against each other in a challenge and trash talk by insulting each other over their Limited Wardrobes. Leshawna says she's sick of staring at Heather's navel, while Heather says she's just as sick of staring at the peaches on Leshawna's shirt.
Totally Spies! starts a trio of high school girls from Beverly Hills, so of course it has bare midriffs all over the place. However, the artists didn't think to give anyone bellybuttons until the third season.
Strangely enough, Beachcomber◊ from Transformers Animated is colorized in a way that makes him look like he's bearing his midriff. Whether this was intentional or not, he's still the only character in TFA to do so.
Most versions of Arcee have a similar look, though it's less prominent in their cases because, unlike Beachcomber, none of them are drawn to look like they're wearing jeans.
Many female Trollz in Trollz, as their bellybuttons have magical gems that are the source of their magic.
Winx Club loves this. Nearly every female member of the main cast wears a midriff-baring outfit as either their default clothes, their transformation clothes, or both. This is later commented on by one of their professors, who expresses disdain for the look:
Palladium: Before we can send you, we have to suit you up in the appropriate attire. You see, the weather on Sparx is quite brutal, so I'm afraid open-toe shoes and exposed midriffs simply will not do. Griselda: If I had my way, this would be the year-round dress code. Faragonda: Oh, Griselda.
That quote was only in the 4Kids dub. It's ironic that out of all companies, this one would blatantly address, given that they've tried to cover it up before, notably giving Nico Robin in One Piece an undershirt...
And then the Enchantix went and took the last one who wasn't in a mini-dress, Techna, and did it to her too. And made the others' clothes skimpier than before.
W.I.T.C.H.'s Will, Taranee and Hay Lin's Guardian forms have exposed midriffs.
Outside of Guardian form, Hay Lin's Season 1 Outfit sorta counts, although you can't see her navel. In Season 2, the girls give up their Limited Wardrobe and each get a couple of trope-relevant outfits.
In X-Men: Evolution, most of the characters changed their out-of-costume Limited Wardrobe every season. Each of the girls had at least one outfit that did this, and all of Jean's did. Jean, Rogue, Boom Boom, Magma, and Shadowcat all did this when they formed the Bayville Sirens as well. Ironically, Jean's Siren outfit bares the least amount of midriff out of all of them.
Artemis on Young Justice as part of her superhero outfit. She wears a more covering outfit when she takes the identity of Tigress.
The new Harley Quinn ditches her jester costume for an outfit that shows her midriff.