Retractable Weapon

So you have a rather long weapon, and you don't have the Bag of Holding or any sort of Hammerspace to store and conceal it. What to do? Make it retractable, thus concealing it and possibly making it small enough to be stored without any problems.

Typically, any type of the Laser Blade is this. Compare Shapeshifter Weapon, where a character that can shapeshift can create many weapons from their body, Morph Weapon which is a weapon that can take different forms and Person of Holding. See also IKEA Weaponry, Impossibly Compact Folding, and Wolverine Claws. Contrast with Collapsible Helmet.

Anime and Manga

 * In Ghost in the Shell: Stand Alone Complex, Paz is shown to carry around a folding kukri which is specific enough to him that.
 * The Guyver has blades that extend out of his arms.
 * Trigun: Vash and his Concealed Alternate Retractable Weapon that doubles as his HAND.
 * Goku's Power Pole from Dragon Ball.
 * In Tokko, Kureha's daggers are shown to be retractable. In the manga version, Sakura's sword is shown doing this as well.
 * In Yaiba, Kojiro's sword has the power to grow longer or shorter at will, though it's still a pretty long blade by itself.

Comic Books

 * X-Men
 * Every incarnation of Wolverine for his adamantium claws.
 * Gambit often carries a retractable bo staff.
 * Batman's Batarangs in some media like in The Batman and Batman: Arkham Asylum.
 * Batgirl (the Stephanie Brown version, at least) carries around a retractable/extendable staff, as does Robin III (Tim Drake).
 * Spoofed in Asterix in Corsica. The Weapon of Choice of every Corsican is a wristblade, which they're not shy to show for intimidation purposes. For the big battle at the end, they happen to have also retractable swords and even rectractable lances—doing exactly the same sound as the wristblades.

Films -- Live-Action

 * Star Trek: Sulu's katana
 * Star Wars: Lightsabers
 * The expanding baseball bat used by Griff Tannen in Back to The Future Part II.
 * Hellboy 2: The Golden Army: had Prince Nuada's spear/sword also with a nifty self repair or slow painful death feature.
 * Tron: Legacy: The Black Guards' staffs and Quorra's sword have Laser Blades coming out of batons (on the same principle as Light Cycles and Light Jets).
 * The Predator has retractable wrist-blades. The sequel Predator 2 adds a retractable staff/spear to the species' arsenal.
 * The Mummy Returns has a retractable spear/sceptre.

Literature

 * Larry Niven's Known Space: variable swords (Monofilament wire reinforced with a stasis field).

Live-Action TV

 * Babylon 5: the Minbari Fighting Pike aka Denn'Bok. Preferred weapon of the Rangers.
 * Andromeda: the Force-lance, which can extend to a 2-meter-long quarterstaff.
 * Power Rangers: Several weapons, sometimes in an impossible manner.

Myths & Religion

 * In Norse Mythology, Thor could shrink down his hammer Mjöllnir so that he could fit it inside his clothes. For this reason, Mjöllnir-shaped pendants are common in that part of the world.
 * Journey to the West: Sun Wukong's iron rod

Tabletop Games

 * In Shadowrun, retractable claws are available as cyberware and known by the nickname "wolverines". As well as a collapsible staff and the Victorinox memory blade which folds into a belt.

Video Games

 * Assassin's Creed: The hidden blade(s).
 * The weapons in Mass Effect fold up to much smaller sizes when you're not using them.
 * Team Fortress 2: The Spy's Butterfly Knife
 * Adam Jensen's arm blades in Deus Ex Human Revolution, which can come out of his wrists or his elbows.
 * Jade uses a retractable staff as part of her arsenal in Mortal Kombat.

Web Comics

 * Homestuck: Nepeta uses a pair of retractable claw-like weapons.

Western Animation

 * ReBoot: Megabyte has Wolverine Claws in his hand. Also, AndrAIa has a extendable/collapsible trident weapon.
 * Robin in Teen Titans has bo staffs that retract to fit in his utility belt. So does Slade.
 * El Tigre has claws that he can use to grab/attack people from a distance, due to a Variable-Length Chain.
 * Optimus' axe in Transformers Animated has an extendable shaft.
 * Wakfu
 * Rubilax isn't so much retractable that size-shifting; its normal form is between the size of a short sword or dagger, but it can turn into a BFS (with Volcanic Veins in option) in a hitch.
 * Some air pirates also have sword-lances that can change size.
 * In Thundercats and its 2011 reboot, the Sword of Omens default size is a dagger/shortsword. When its wielder utters a battlecry to unleash the power of the Eye of Thundera (also known as the Warstone in the reboot), the Sword becomes a longsword. The 2011 reboot explains the Sword's small default size:

Real Life

 * Truth in Television: Expanding batons, switchblades, butterfly knives, swiss army knives, and firearms with folding/clasping stocks.
 * Many predatory animals have retractable claws.