Secret Diary

One character finds the secret diary of another character (almost always female), that contains all their darkest secrets and proceeds to read it and/or pass it around their friends, etc. Naturally, the character who owns the diary is furious when they discover this invasion of their privacy.

Advertising

A commercial for AT&T U-Verse has the family seated around the dinner table. The son is reading the daughter's diary, which the parents have also read. The daughter is quite pleased. (The theme of the commercial is that having U-Verse makes everybody happy.)

Anime and Manga

Slightly different set-up in Patlabor: Lt. Clancy is preparing a report on the rest of the team before leaving the force, and the others sneak into her apartment to read it.
Tsukihime: Shiki Tohno finds Makihisa Tohno's hidden diary, in which it is revealed that the Tohno blood line contains demonic blood, that Akiha is one of those who has demonic traits, that Makihisa himself had a split personality, that Kohaku and Hisui are adopted synchronizers who can calm the demon blood by exchanging bodily fluids and that there was indeed a third child with the same name as Shiki that was adopted after Makihisa wiped his clan out.
The manga Wandering Son has a shared secret diary between the eleven-year-old main characters Shuichi and Yoshino, in which they write to another about their various Crossdresser experiences. When a boy gets ahold of the diary and passes it around, both kids in general and Shuichi in particular are ostracized. They start a new one not too long after.
In the ending to the anime version of Tona Gura, a pivotal diary that Hatsune gives Kazuki is Kazuki's own forgotten childhood diary which reveals that Yuuji's always been playful; teen hormones now make that playfulness obnoxious and immature, rather than cute. She also sees in it that she herself used to be a great deal more playful, but that she has also not outgrown her childish tendency to want complete control over how things are, especially with Yuuji. This marks a huge turning point.

Comic Books

One Archie Comics story had Betty losing one of her diaries (to be more exact, the one she wrote about her moments with Archie in) and was worried that Veronica would get her hands on it. To make matters worse, Veronica overheard her and offered $100 to anyone who turned it in to her. Fortunately for Betty, Mr. Svenson returned it to her before anyone else could find it.

Fan Works

The recommended Ranma ½ fanfic Genma's Journal has Nabiki finding Genma's secret diary and discovering that Ranma's father is actually a Magnificent Bastard and hiding behind Obfuscating Stupidity rather than the doofus he appears to be.
In Real Person Fic, Just Taken, Melanie starts a secret diary, complete with writing in Klingon, on the information from a fellow patient at the Bedlam House, she found herself in.

Literature

The disruption in Harriet the Spy.
William Sleator's Others See Us uses two stolen diaries as the MacGuffin, with the the inversion that the thief didn't steal them to read them, because she's telepathic and knows the contents already.
In Harry Potter and the Chamber of Secrets, Draco Malfoy plans to do this when he finds Tom Riddle's diary among Harry's possessions and mistakenly thinks it's Harry's diary.
- According to Word of God, the way Riddle's diary uses Ginny is a serious Deconstruction of this trope, foregoing others reading the diary and instead making the diary itself a malevolent entity. J. K. Rowling finds diaries to be "really, really frightening" due to her sister confiding her innermost thoughts to a diary only to worry about people reading.
The book Click Here to Find Out How I Survived Seventh Grade is somewhat centered around this trope.
Subverted in the Liavek story "Paint the Meadows With Delight", where the main character steals her sister's diary in hopes of figuring out why she's been acting so strangely, and, since she can't read the language it's in, tries to get an older relative to translate it for her. It says much about the family involved that Jehane did't realise what she had done was objectionable until she got called out on it.

Live-Action TV

It is almost guaranteed that a Teen Drama series will have an episode centered around this at some point.
In House, House steals his ex-common law wife's psychiatrist's notes (mostly about how her relationship with her husband sucks and she has the hots for House again) and recites them to Wilson a few days later.
Red Dwarf: Rimmer walks in and finds Dave "waits for the film to come out" Lister reading. When he asks what the book is, Lister flat-out states that it's Rimmer's diary, then repeats some choice excerpts. Then:

Rimmer: I don't believe it! You've been reading it out to the Cat?!
Cat: Only the best bits!

- Mild subversion here is that Rimmer isn't really surprised Lister's reading his diary: he's upset he's doing it publicly.

Lister: So you've read my diary.
Rimmer: Yes, but at least I have the common decency to do it sneakily behind your back.

Titus: Teenage Christopher comes home to find his father reading his diary to his poker buddies.
One scene in Spaced shows Tim reading Daisy's diary: "Ha, ha, ha, thrush!"

Newspaper Comics

One wonders why Paige Fox of FoxTrot even bothers trying to keep a diary when Peter and Jason constantly read it openly. Jason has go so far as to post excerpts from her diary online and has even written in it (apparently on multiple occasions) a confession to really being an ugly alien being.

Video Games

Illusion of Gaia: "Kara's diary is secret. Read it?"
One mission in Bully has you recovering a nerdette's diary before a teacher publicizes it.
In the sixth chapter of Paper Mario: The Thousand-Year Door, a ghostly Toad asks you to get his diary from the baggage area and threatens you against reading it. The game gives you the option to read the diary anyway, and after asking things like "wait, seriously really?" and "OK, let's think this through for a little bit" you actually can read the first page before the ghost appears and kills you.
- In the first game, Peach actually can read the secret diary, and indeed she has to in order to advance the plot. It belongs to Bowser.
In the Trauma Team game for the Wii, there's a Forensics chapter. The third case gets incredibly disturbing when the diary of Alma is found, and it depicts her descent into insanity, due to her prefrontal lobe tumor, from a loving mother and wife to a psychotic 'Beast of the Lord' who wants to kill her daughter and paraplegic husband in order to let them all go to Heaven together.
In Animal Crossing: Wild World for DS, a snooty female character sometimes finds(buried underground) diary owned by someone else (most likely a normal female). She asks you if it is ok to read that. If you say so, you are asked where to start.

Western Animation

Hey Arnold!!, "The Little Pink Book": Said book is Helga's diary. Helga removed and ate the page revealing her name, just before Arnold would have gotten to it.
Ed, Edd 'n' Eddy, "For Your Ed Only": Eddy finds Sarah's diary, and Ed and Edd try to return it before Sarah finds out they peeked and beats the living daylights out of them.
Code Lyoko, "Logbook": It's all made worse by the fact that Ulrich, the diary's writer, mentions XANA and Lyoko.
- Later repeated with Yumi's diary in "Lost at Sea".
- And then again with Jérémie's logs in "Echoes". Really, one has to wonder why they keep writing these things down.
In Jem, the Misfits get Kimber Benton's diary and use it against her. Luckily, she kept her Cool Big Sis secret.
In an episode of Doug, the main character's journal gets taken and he spends the entire episode trying to get it back, only to find that school bully Roger Klotz had found it. Fortunately for Doug, Roger was unable to read his handwriting and get at his secrets.
In Total Drama Island, Heather does this to Gwen.
Arthur "Sue Ellen's Lost Diary" Sue Ellen loses her diary at the library. Arthur and the others find it and are tempted to read it, but decide to just give it back to her instead.
In The Weekenders, Carter peeks at Tish's diary and finds out that she's moving away. It turns out, though, that it wasn't her diary he found—it was her novel, "Diary."
In 6teen there is an episode where Jonesy finds Jen's diary at the lost and found. Kaitlin starts reading it out loud at the fountain.
Done in The Fairly OddParents with Timmy reading Vicky's secrets and Wanda helping Timmy just because in the diary Vicky says that "the pink squirrel is fat". And Cosmo is not happy with them doing it.

Cosmo: A diary is where a girl...And me...Can express their feeling without being judged.

An episode of Family Guy had the family, except Meg, gather in the teen girl's room while Lois regaled them with pages from her diary, to everyone's laughter. Enter Meg, who proclaims she hates them and runs out of her own room crying. In response, Peter cracks open a beer and encourages Lois to continue reading.