Cobweb of Disuse

A cobweb or two is often used as a sign that something hasn't been used or hasn't moved in a while. This is a standard part of the decor in Haunted Castles and Haunted Houses, so much so that white fibrous decorations simulating cobwebs appear in stores every October.

In Real Life, this is only partially true. A spider only needs a few hours to build a web on something. On the other hand, not regularly removing cobwebs from something will allow dust to build up on them, making them a lot easier to see.

Taken to extremes, this can become a Cobweb Jungle. Compare Wallet Moths. See also Trash of the Titans and Extremely Dusty Home.

Anime and Manga

 * Ranma ½: Happens within minutes to the Tendou household when they temporarily lose all their money due to shenanigans involving Nabiki.

Comics

 * Mad Magazine did this a LOT, particularly Sergio Aragones' "A MAD look at _____". If a person bought something that sits in disuse, you'll see it sitting on a shelf or in a closet with spider webs.

Fan Fic

 * Present in Thunderstorm's ice cave in Calvin and Hobbes The Series.

Film

 * Played with in the 1931 Dracula, where much of the Count's castle is swathed in cobwebs that make it appear totally deserted. At least, they seem to imply nobody's been using it ... until a sneaky camera cut makes it appear that the vampire has walked straight through a large orb web without disturbing it.
 * The chapel in The Legend Of Hell House has these, especially over the crucifix. Since the house's previous inhabitants were notoriously irreligious, the trope is apropos.
 * In Who Framed Roger Rabbit?, Eddie keeps the desk of his late brother and partner Teddy untouched since his death as a Shrine to the Fallen. It is covered in cobwebs and layers of dust.
 * The Indiana Jones movies had this, what with all the ancient ruins and stuff.

Literature

 * In the Thursday Next books, Thursday mentions the cobwebs at Satis House when she goes there to meet her Jurisfiction Mentor Miss Havisham.
 * Jenna's Palace in Septimus Heap is filled with cobwebs, mostly due to the staff being overwhelmed by the sheer size of the Palace and only a few parts being truly used.
 * In The Mallorean, Salmissra aims a Take That at Polgara when the snake queen says that the long centuries have filled the sorceresses wits with cobwebs.

Live Action TV

 * In the beginning of the BBC series Sleepers, about a couple of KGB agents sent to be sleeper agents in the UK in 1966, the present day (1990) KGB discover the bricked up, unused facility where the agents were trained, full of mid-sixties British pop culture and cobwebs.

Religion

 * Invoked by Allah himself in one of the very few miracles stated to have happened around Muhammad is, when he was escaping the city of Mecca and the Qraysh who were out to kill him, that he and his friend Abu Bakr hid in a tiny cave. Their pursuers followed their track to the entrance of the cave, but in the meantime a spider had woven her web all over the entrance and a dove had set her nest there. They decided that clearly no-one had gotten in there, and resumed their search elsewhere.
 * A similar tale is also told about King David (hiding from Saul) in the Judeo-Christian tradition.

Tabletop Games

 * The second edition of Dungeons and Dragons had an illusion like this, to confuse burglars and thereby protect valuables.

Video Games

 * In Psychonauts, parts of people's minds which haven't been accessed in a long time are blocked by "mental cobwebs" which you need to buy a specific piece of equipment to clear.
 * In Minecraft, poisonous spiders have webs in abandoned mine shafts.
 * As one of its first puzzles, Anchorhead has a cobweb standing between you and an intriguing-looking iron key in the corner of the Verlac mansion basement.

Webcomics
"Rachel: It's a happy spider! Surely that counts for something?"
 * El Goonish Shive had a dojo disused but lacking cobwebs... then the sketchbook explains why.
 * Scary Go Round got it on the very first page:

Western Animation

 * In the Scooby Doo episode "What the Hex is Going On?", the old Kingston Mansion has these in the background (corners, connecting furniture to walls). Also present in the Vasquez Castle (notably on the portrait) in "Hassle in the Castle".
 * The Simpsons: When the family goes to the library to do research for school they find no books and cobwebs on the shelves. So Marge tells them stories of Henry VIII, Sacagawea and Mozart.
 * On the SpongeBob SquarePants episode "Pretty Patties", the Krusty Krab is covered in cobwebs when business gets slow. SpongeBob tries to clean one, a spider makes another one. Later, Mr. Krabs himself is seen covered in cobwebs.

Real Life

 * When the Nazis invaded France, many French people would hide their valuables in their wine cellars by bricking off part of the cellar. They would then find spiders and place them in front of the newly built walls so they would build webs and make the walls look older. (In a similar vein, wine merchants would often run a scam against Nazi officials by sprinkling dust on top of bottles of cheap crap to pass it off as old and valuable.)
 * Speaking of wine and cobwebs, wine bottles are possibly the only food-item that you'd want to be delivered to you with dust and cobwebs all over it.
 * Invoked by Halloween cobweb decor.