Tan Lines

Exactly What It Says on the Tin, Tan Lines occur when the human body gets tanned in areas of exposed skin but the places previously covered during the tan look noticeably lighter. They are usually seen as a way to make a character look funny after a bad or flawed tan job. Other forms include the farmer's tan (face, neck, and both arms, from being out in the field while wearing a T-shirt or overalls) and the trucker's tan (face and one arm, from the sun shining through the windshield on the face, but with the pale arm inside the cab and the tan arm out the window).

There are some who find tan lines, especially if the contrast is strong, to have an erotic edge, though tan lines rarely seem to be used much for Fanservice.

Examples of Tan Lines include:

Advertising

The girl on the Coppertone suntan lotion bottle has a white butt but tan everywhere else.
A man on the "Bud Light Cruise" commercial has tan lines that indicate he'd been wearing a woman's bikini.

Anime & Manga

In the obligatory Beach Episode of Tenchi Universe, Kiyone develops a classic farmers tan while working as a life-guard. When she shows off her swimsuit at the swimsuit contest later in the episode, she's initially embarrassed by the farmers tan- until the crowd goes wild, all of them finding it sexy.
Kagura of Azumanga Daioh, during the summer arcs (she's on the swim team and wears a one-piece for the rest of the year, so it shows up when she wears a bikini; she also shows Osaka, but not us).
Chika from Ai Yori Aoshi normally lives at the beach, so she has them following the bathing suit area.
K-On!: This happens to Azusa every time there's a Beach Episode. It's lampshaded by everyone, every time.
Momo from Peach Girl got tan lines along with her bleached out hair from swimming for school.
After returning for the beach during an episode of Beelzebub, Oga admires the nice tan he got...only for him to discover the Beel shaped tanline on his back.
Rio -Rainbow Gate!-: Linda has tan lines around her chest that are displayed by her skimpy top. Given that she's a Ridiculously Human Robot Girl, one can only presume that they were deliberately designed to be there.

Comics

In part of the recent X-Men Curse of the Mutants storyline, apparently the technology protecting the vampires from sunlight also protects them from Dazzler's powers. We see a flashback of Emma Frost talking to her, Frost admonishing Dazzler for having given her tanlines.

Films -- Live-Action

In Police Academy 5: Assignment Miami Beach, Harris is tanning on the beach. Nick writes "DORK" in sunblock on his chest. When he wakes up and walks on the crowded beach, Hilarity Ensues.
In Spencer's Mountain, Clayboy goes up the mountain with his girl-friend for a picnic, and comes back with a sunburn on his back. His father helps him put lotion on the burn, which goes much further down past his waist line than it would have if he had kept his pants on.

Music

Mentioned in the chorus of Kenny Chesney's "She Thinks My Tractor's Sexy".

Print Media

There's a classic Mad Magazine cover where two boys are showering and have the usual tan from swimming, i.e., white butt and tanned everywhere else. Alfred E. Neuman's tan, however, is reversed.
The Onion had a picture showing a woman with tan lines zigzagging over her body in nonsensical patterns.

Web Comics

It is possible for milk teeth to get burnt as My Milk Toof proved in the Beach Episode.
A strip from The Perry Bible Fellowship shows a guy getting a white silhouette of his secret girlfriend where she's blocked him from getting tanned. I think you know where I'm talking about.
This strip of Luke Surl.
The subject of this Buttersafe comic.
When Spinnerette got a tanline around her mask, her roommate Sahira initally laughs at the panic the former was undergoing, until it's pointed out that this would undermine her Secret Identity. Sahira then offers to help her with using makeup to cover it.
User Friendly: even Great Old Ones are not immune.

Web Original

In The Saga of Tuck the protagonist ends up with some nice tan lines caused by strappy swimsuit. "He" has to go to lengths not to let others see them.
CollegeHumor presents a few other variations...
Strong Bad in the Strong Bad Email "suntan" from Homestar Runner.
From The Onion, "Woman's Tan Lines Don't Make Any Sense"
Other Girl-sempai/Sunglasses from the Creamsicle internet meme has tan lines around her crotch.

Western Animation

King of the Hill is notorious for having almost all the white male characters having untanned portions of skin when they take off their shirts. Boomhauer, on the other hand, consistently has a dark tan (he has a tanning bed in his bedroom).
In Rocket Power, former new guy Sam had one when he moved to Ocean Shores, even though his body should have evened out by now.
Owen and Harold from Total Drama Island have tanned arms, legs and face, but has pale upper arms and stomach. Ezekiel has much of the same, except his legs are pale.
Hawkeye from The Avengers: Earth's Mightiest Heroes, has a farmer's tan while he sits out in the sun in episode 20. That episode marked the first time in the show when he wore something other than his superhero costume.
Howie, the lead character of Almost Naked Animals, combines this with Fur Is Clothing. Like the entire cast, he wears only underwear and has most of his fur shaved off, but there are tan lines on his bare skin.
That fat man who constantly loses his ice cream cone every single time in Lilo and Stitch.
Eddy from Ed, Edd 'n' Eddy has tan lines around his eyes.

Real Life

In the early 1950s, a story began to circulate about a boy in Syria who had allegedly been raised by gazelles. The story was debunked partly because photos of the nude boy showed him with a farmer's tan.