Both Edison Lighthouse and Eleanor, in Groovy, Kinda, favor ultra-low hiphuggers with at least a bit of tummy showing. The drunker Edison gets, the more she shows. She also has a mysterious navel piercing.
Any's super-heroine costume does this in M9 Girls!. She seems shy to be seen like that the first time she wears the costume.
Zeetha of Girl Genius has many outfits with this look. Of the five she's worn for more than a couple of pages, three of themshow hermidriff. The other two technically belong to other people, which explains why they don't.
The Noordegraaf Files: Has Akila, a Nature spirit. As she comes from a race where wearing clothes is unheard of, it's not that hard to understand. If you lived all your life without any clothes on, and one day people (whose customs you don't understand )made you wear some, how would YOU feel?
In Terinu this is Gwen's default condition when she's not wearing her CyberGlider uniform. Though almost every female in the main cast (Joleen excluded) shows their bellybutton at least in the gallery pictures.
Comparatively common in fashion in the Drowtales world along with detached sleeves, especially among the Sullisin'rune clan. When both Quain'tana and Ash'waren showed up with bare midriffs in one scene (spoilers!) the fandom nearly imploded from the Les Yay, and a poll was set up to see who had the best belly (Ash'waren won).
Kin from Goblins. It looks like Goblinslayer was forcing her to wear that, because as soon as she practically can she grabs a modest neck-to-waist coat.
Angelika from Our Little Adventure, though lately she's donned a chain shirt that doesn't bare the midriff. She doesn't like the new armor much, though. She 'returned to the classic costume design' when they returned to Huckleton. She now wears a Mithril chain shirt that looks like leather and is brown.
When she's not wearing a painted-on black dress, Miriam from Out There often favors a tank top or T-shirt that extends down to about mid-ribcage. Her friend Sherry, who usually dresses far more modestly, occasionally rocks the belly button as well. 
In El Goonish Shive, both Susan and Nanase wore this kind of outfit in early comics. Now only Nanase wears it anymore.
Most of the cast of Ménage à 3 (of both sexes) are prone to flashing quite a lot of flesh on screen, midriffs included; for example, lead character Zii makes her very first entrance in a crop top. But the specific style is probably executed with the greatest effect by Senna. Well, she is a lingerie model.
During the period in the '00s when it was fashionable for women's shirts to stop at or just above the waistline, most of Questionable Content's female cast reflected this on a regular basis. Nowadays it's only occasional, such as here.
Robin DeSanto's default top is a short orange shirt; at one point she lampshades the exposure by telling Joe "Check out these abs!"
Sal's preferred look when she's in alien-fighting mode; her casual clothes bounce back and forth on whether they do.
Joyce normally wears a SEMME T-shirt, but her hypersexualized duplicate Anti-Joyce cuts away everything on her shirt except the yellow stripe across her breasts. Later on, when the real Joyce has a bit of a breakdown and decides to let herself go, she dresses just like Anti-Joyce.
Kat Vance from Phillip M Jackson's peculiar webcomic Sequential Art wears sensible outfits at work, in winter or at social functions, but she likes to bare her midriff while at home with her flatmates, during the warmer months, and while out with friends. Since Kat is an example of Petting Zoo People, her fine feline physique is one worthy of display.
Chrys's shirt in Bugged Run always shows at least some of her stomach.