Notable Trailers

A trailer is basically a short advertisement for a film. Usually borrowing small pieces of the film, it can be a problem to make one, sometimes harder to make than the movie itself. A good trailer encourages people to go see the film (or to rent or buy it if it's on DVD), but at the same time shouldn't give away too much of the film, and to be honest, should not promise things that are not in the film (with some exceptions; sometimes, to establish the conditions in a film, they'll add a scene of a few seconds that isn't in the film because the conditions existing are obvious and don't need to be repeated in the film.)

There have been some notable trailers where the trailer was an accurate representation of the content of the film, and neither lied nor spoiled the film. And perhaps some are notable for either being tremendously better than the film, or being tremendously worse.

Film

 * Cloverfield :A teaser trailer shot in a handheld camera style (like the rest of the movie) that appears to be a lighthearted party flick before an abrupt attack by a giant, completely unseen monster and ending with the iconic shot of the Statue of Liberty's head rolling in the street--and no title. All that is shown is the director (JJ Abrams) and the release date (1-18-08). The trailer kicked off one of the most secretive viral marketing campaigns in movie history.
 * Bonus point for the entire trailer being shot before the rest of the film. Even the party scenes were re-shot for the final version after the trailer had already aired.
 * The Blair Witch Project went viral back in ye olde days of 1998, sending chills down the spines of college students who showed it to each other huddled over CRT monitors, because you couldn't embed videos back then.
 * The trailer for Independence Day had basically a terrific Money Making Shot: a scene of the alien mother ship vaporizing The White House. And in case that failed: "Welcome to Earth.*punch*"
 * The trailer for The Matrix shows Morpheus running across the roof, jumping the huge chasm and landing (heavily) on the other side.
 * The trailer for The Lord of the Rings: The Return of the King was excellent, wasn't it?
 * "It's quiet now . . . they came from the blackness . . . "
 * "You threaten my people with slavery and death." "This is madness!" "Madness... ~This! Is! SPARTA!~" **Badass kick! Cue Crowning Music of Awesome!**
 * The trailers for Watchmen have been consistently making nerds cream themselves since they began getting trickled out of Warner Brothers. For others, however, the trailer has been something of a turn off. Comments such as "lol suuuuuux looks liek a batman ripoff!" and "omg, that blue guy is just a silver surver ripoff..." have been all too common on the internet.
 * The Screamer Trailer for Suspiria, which told you nothing about the plot of the movie itself.
 * The Fly. Be afraid. Be very afraid.
 * The trailer for Taken was mostly made up of the protagonist's Badass Boast to the kidnappers who had taken his daughter. It worked.
 * The teaser trailer for Garden State was incredibly gorgeous and near perfect, and it signaled the rise of Pitchfork Media-beloved/hipster music in indie movies. The song used is "Let Go" by Frou Frou, Imogen Heap's old band.
 * Sin City had a incredibly memorable trailer that showed off the amazing art style, the motivations of the characters, the rough story and basically just told the viewer "yes, we are trying to make this as faithful to the material as possible". It also had an amazing song in the trailer, an instrumental version of "Cells" by The Servant. Demand for the song was so high that The Servant was forced to release the instrumental version. The music can also be heard on the special edition DVD menu.
 * When Alvin and The Chipmunks got a live-action film, it's a Foregone Conclusion that if it got a sequel, the Distaff Counterpart group the Chipettes would have to be in it too. This trailer proves our assumptions are correct.
 * The trailers for The Phantom Menace and Revenge of the Sith were both massive hits, with many Star Wars fans purchasing tickets to Meet Joe Black for the mere opportunity to see TPM's trailer.
 * The trailer for Citizen Kane is almost as distinctive as the movie itself. Starting with a startling shot of a microphone approaching the camera, Orson Welles discusses the main character in voiceover, accompanied by scenes of the supporting characters give their opinions on him, along with candid footage of the actors at play. Notably, there is almost no footage from the film itself, and the main character doesn't get so much as a glimpse.
 * Sometimes, Alfred Hitchcock would appear in his movies' trailers, and make tongue-in-cheek promotions of the films.
 * Probably the earliest example involved Hitchcock suggesting the viewers take a vacation with North By Northwest, promoting a peaceful cross-country tour against clips of the leads in peril.
 * The trailer for Psycho has Hitchcock giving the audience a tour of the Bates Motel, hinting at the horrible events within.
 * The Birds had a trailer, showing Hitchcock giving the history of man's relationship with birds. He tells us the horrible things man did to birds in a deadpan manner, culminating with the bird in his cage biting his finger ("Now why did he do that?").
 * The trailer for Star Trek not only featured no footage from the movie itself, it didn't reveal that it was a trailer for Star Trek until Leonard Nimoy's voiceover: "Space...the final frontier..."
 * The trailer for Alien didn't reveal much of the plot, but everyone were too scared to notice.
 * Brian de Palma's Blow Out had a remarkably modern, fast-paced trailer for its time (1981) -- not to mention a really memorable one.
 * The Toys trailer had Robin Williams introducing himself and the wheatfield soundstage from the movie, then starts goofing off and not actually describing the film. "What's it about? It's about an hour and a half."
 * The teaser for Super 8 consisted entirely of a truck smashing into a train, something breaking out of a boxcar, and a few frames to kick-start a viral marketing campaign. Because of its ambiguity, it spread across the Internet like wildfire.
 * The teaser for the American version of The Girl With the Dragon Tattoo was a series of dark scenes with no dialogue and was scored to a Led Zeppelin song by Trent Reznor and Karen O. It became one of the most iconic videos of 2011 and inspired numerous loving parodies and much admiration. It can be seen here.
 * The Hitchhikers Guide to The Galaxy goes meta.
 * Watch the first trailer for Terminator2 and you just HAVE to see it the day it comes out in the summer...of 1991! Note how it consists completely of footage made exclusively for the trailer without showing any actual movie scenes and only has one line of text. It definitely gets you pumped.

Video Games

 * Super Smash Bros. Brawl has the Nintendo World 2006 trailer, the E3 teaser, and the Subspace Emissary trailer. And those are just the trailers that are not included in the game's movies section.
 * The internet, and half of the games industry, shat itself in amazement at the Metal Gear Solid 2 trailer, all the more so because dial-up internet was just coming into popular use and nobody could actually get their hands on it. The reaction probably cemented the importance of good trailer editing in the games industry from then onwards.
 * Continuing down the video game theme: no matter what you think of the series, the epic trailer for Halo 3 at E3 2006, complete with the orchestral song "Finish the Fight," is sometimes seen as a milestone for cinematic video game trailers.
 * Dead Island went from a virtually unknown IP to one of the most talked about games on the Internet literally overnight thanks to its trailer (which didn't contain even one frame of actually gameplay footage).
 * This trailer for Mass Effect 2, composed entirely of clips from the game plus one or two extra lines. The music is one of the coolest things in existence.
 * The Destiny traler for Dragon Age 2 is interesting, since it's a scene from the game re-imagined in a much more epic manner, with Flemeth being Flemeth in the voice-over.
 * Similar, this fan trailer recreates the rendered intro movie of FreeSpace by using the FreeSpace 2 ingame engine with all the graphic updates and mods they created. And it by far surpasses the original.

Western Animation

 * The Toy Story 3 teaser trailer had Woody and the toys build a makeshift logo for the movie, but Buzz comes in and announces that he made a superior logo overnight.
 * The Incredibles teaser, with Mr. Incredible having some trouble with a belt. It even got to the TV spots, when it's not in the movie.
 * Pixar has a long history of teaser trailers made with original material, starting with A Bugs Life.
 * The original trailer for The Lion King consisted of just the opening (musical) sequence, "Circle of Life". Disney would later try slight variations of this, with one Pocahontas trailer showcasing "Colors of the Wind", and a Dinosaur promo featuring the silent opening.
 * Lilo and Stitch had a memorable ad campaign in which Stitch showed up uninvited in scenes from Beauty and The Beast, Aladdin, The Little Mermaid, and The Lion King.
 * The Super Bowl Special commercial for Rio played like an ordinary commercial until the end, where during a clip of Rafael knocking out the last of the monkeys, a hint for a Rio-related Angry Birds contest is briefly shown on a box in the background.

Web Animation

 * The "Red", "White", "Black" and "Yellow" trailers for Rooster Teeth's RWBY were beautiful, filled with awesome music and generated an incredible buzz for the series starting months before its first episode.