Visible Sigh

Relegated mainly to lighter anime, the Visible Sigh is a little white cloud that appears near and moves away from the mouth of a person sighing. It is also used to symbolize heavy breathing after exertion and some cases where the onomatopeia "phew!" is used in English. The cloud itself is usually circular with a small "stem" that points toward the character emitting it.

In many an anime, the cloud will be compacted, resembling a mushroom.

Anime and Manga

 * Sailor Moon.
 * Abenobashi Mahou Shoutengai.
 * Magic Knight Rayearth.
 * Dragon Half.
 * Fullmetal Alchemist (Image.)
 * Travis Willingham, Roy Mustang's voice actor, once made a comment on this at a convention ("Ed just burped out a mushroom!").
 * Like so many other anime tropes, Teen Titans was fond of using this, particularly with Raven.
 * Manga example: Yotsuba&!
 * Haré+Guu.
 * Pretty Sammy
 * Ninin ga Shinobuden
 * Kirby does this when he lands after floating, and can hit enemies with it.
 * He also gets one in an episode of the anime series, shaped like an apple.
 * Sketchbook
 * Pokémon, including by Pokémon, mostly Pikachu when he's forced to play Team Dad to younger Pokémon.
 * Mahoraba makes frequent use of this, even in the opening.
 * Rukawa Kaede in Slam Dunk does this all the time, especially when confronted by rival Sakuragi Hanamichi.
 * There She Is, when Nabi sees that Doki's cake is unscratched.
 * Lucky Star but there it's more like a speech bubble cloud.
 * Haruhi-chan
 * The page image comes from Happy Lesson. Show us how it's done, Mutsuki-sensei!
 * Vampire Knight
 * In Pita-Ten, Misha sees Takashi sighing complete with a little cloud. She then mentions the Japanese saying that some of your "happiness" escapes whenever you sigh. When this causes Takashi to sigh again, Misha grabs the cloud saying she can see the happiness getting away.

Video Games

 * In Animal Crossing: Wild World.
 * Kirby exhales after he's been using his flight abilities, and the cloud can hurt enemies. Whispy Woods, Gooey, and King Dedede do this, too.
 * The original Bubble Bobble arcade game while the player character gets incinerated by an electric bolt by a Space Invader.

Web Comics

 * Gunnerkrigg Court, Kat in Chapter 35, then Annie in Chapter 36 when discovering she didn't mess up the rest of Red's life by accident. For extra expression, supplied with the closed eyes and a weak smile.

Real Life

 * The magic of condensation will allow you to pull this off on a cold Winter day.
 * And the magic of Tim David's magic trick "Solid Breath" will allow you to do that anywhere, anytime.
 * Ditto if you drink anything very cold (Like say a fresh glass of milk) and sigh afterwards.