Boxers with red/pink hearts

These seem to be Chuy's underpants of choice in Girly.
In this Nodwick comic, Artax gets busted for not wearing guild-approved midnight blue underwear with stars and moons. He owns at least one set of pink with hearts.
A very rare female example: Fuschia in Pastel Defender Heliotrope.
Captain Phi of the Royal Guard has some nice ones in Exiern.
Freefall: Kornada's boxers, shown when Sam swipes his clothes to use as a disguise for Florence.

Sandra and Woo involved this trope. When Ralph's pants get sliced off by Cloud, the last panel shows him in his heart boxers. In one of the comics, Woo literally tears off Sandra's clothes, including her shirt. All that's left on her is her underwear with hearts on them.
In the very first panel of Allen the Alien, Allen wears this type of boxers.
Hero wears these in RPG World (as seen when he opens his inventory).
Burk in the now defunct A Path to Greater Good, as seen whenever he loses his pants.
Cucumber Quest's Commander Caboodle has pink boxers with I Heart Justice printed on them.
Curtailed's Fox wears heart-print boxers, and nothing else, to a job fair in this strip. According to the rant, the real-life Fox actually owns at least one pair of these.
The title character of Tony Comics sometimes wears pink boxers with white hearts.

Other-Print boxers

Apparently Professional Killer Black Monday Blues has a pair of boxer shorts that have a fish-pattern printed on them.
This strip of Ghosts of the Future shows Silver wearing boxers with dinosaurs printed on them.
Post-reboot, Burk in Hero Oh Hero is still losing his pants, but he now wears polka-dot boxers of various colors.
In this Freefall strip, Sam comments mentally that the "Teddy bears in space" shorts he's wearing may not have been the best of clothing decisions..
Girl Genius:
- An omake shows that Gilgamesh apparently had "MAD" emblazoned on his boxers (in the world of GG, "Madboy" is slang for a Mad Scientist, which Gilgamesh certainly is).
- There is also a Wulfenbach sigil, like on everything else.
- In another week of omake, we find that Tarvek has "BAD" on his.
Drecker from Adventurers! had dagger-print underwear.

Mr. Mighty from Everyday Heroes has boxers with his logo.
Questionable Content:
- Marten has pink boxers with red polka dots.^note
- Angus has embarrassing drawers though we don't see them.
Nerf NOW!!!!: Appropriately enough, the BLU Pyro sleeps in blue flame boxers.
The Inexplicable Adventures of Bob!: Rocko Sasquatch in polka-dots (Bob needs Brain Bleach).
In Blank It, Aric wears polka-dot boxers and Lemmo wears Space Invaders boxers.
Here in +EV. Poker-themed, of course.
Bill spends an extended scene of Fletcher Apts Chained to a Bed wearing only indigo polka-dot boxers.
In Monster of the Week, Fox Mulder's boxers have Grey faces.
Homestuck:
- John wears boxers with a printed pattern of green slimer ghosts.
- Terezi's scalemate boxers, overlapping with Childish Women's Underwear.
Prezleek Comics: Prez has been shown wearing boxers with Quest Icon print and with yak print.

Bennie in A Doemain of Our Own wears briefs with steaks and bacon print. When he discovers a rose on Sue's undies it inspires a classic pun.
Axe Cop wears underwear with an animated picture of himself chopping off a bad guy's head. He really doesn't do anything halfway.
The King of the Franks in Scandinavia and the World is shown wearing Hello Kitty print underwear.

The Wotch: Anne Onymous wears Wonder Woman Underoos.
Angela in Punch an' Pie also has a set of Wonder Woman Underoos. It just belays her Womanchild personality.
Somewhat common in Questionable Content', though usually off screen:
- Marigold wears frilly Badtz-Maru panties.^note
- Faye seems to have quite a collection. She would never wear leopard print underwear. Only jaguar print. Or monkeys. Or dancing alligators.
In another Nerf NOW!!!! example, the RED Sniper is too distracted by Engie-Tan's strawberry panties to notice the BLU Pyro sneaking up behind him . . .
Chloe in Eerie Cuties — pink, frilly, and with baby chicks. Probably not something you'd expect from a succubus. Then there are Doom Panties, with only one little skull with a bow for ornament, which probably elevates them to Happy Fun Ball status.
Hatsuki of Moon Over June has a huge collection of these and often wears them around the house.
Marie of Sandra on the Rocks wears Super Mario Bros. "mushroom" lingerie. Well, she is a rather dedicated Girl Gamer.^note
Baskets of Guts: Rikleeno's underpants are dotted with bourbon lilies.