Hard to believe that Batman's utility belt can hold anything more than just credit cards and a pack of gum. In responding to a question over at ComicVine (a go-to comics encyclopedia), Zachary Davidson of Quora replied with the following list. So if you're planning on going as The Dark Knight for Halloween, you might want to stock up on these first:
Batarangs: Shuriken or boomerang-based throwing blades made to resemble the symbol of a bat. Some alternate versions of his Batarangs explode, emit electrical blasts, and sometimes come with detonators.
Grappling Hook: Also known as the Bat-Claw. Batman can use this to grapple onto high locations or can grab hold of a criminal by directing it at him. He sometimes uses it to catch run-away street thugs that cannot put up a fight against him.
Gas Mask: This is used to protect Batman from dangerous fumes and other airborne chemicals. This fits over the bottom half of the mask.
Re-breather: Allows Batman to breathe underwater or in a vacuum.
Night Vision Goggles: This allows for Batman to see in dark or dim-lit areas. It also grants him the ability to view the infrared spectrum. (Now built into his mask.)
Sonar: Used in his mask to locate criminals.
Tracers: Bat-shaped signal tracers that can adhere to most if not all surfaces.
Communicators: Batman communicates with different members of his team and also with other superhero allies using closed-circuited signals.
Lock Picks: Batman uses these to open locked doors.
Cuffs: Special cuffs that only few enemies and heroes can break out of. Batman can quickly lock a enemy to a pole or deliver him to the police.
Bolas: Batman swings these Bat-Bolas over his head to wrap up an enemy.
Smoke Grenades and Pellets: Batman can throw these grenades in order to use a stealth technique, distract an enemy, escape a situation, or plan his next course of action.
Sonic Grenades: These disrupt electronics and disorient enemy hearing.
Flash Grenades: This gadget is used to temporarily blind enemies.
Mini-Mines: Mines that can fit in any small area or used as low-grade explosives during combat.
Thermite Explosives: Capable of burning through most materials.
Plastique: Small exploding devices that have a timer or detonator switch to activate.
Tasers: Batman carries this to shock or neutralize an enemy. This weapon was often used in the Golden Age/movies.
Brass Knuckles: Used to engage an enemy with more force behind his blows.
Tactical Knife: Batman does not use this in combat, but he has used this to cut through many durable materials.
Mini-Computer: Batman carries two of these. One is located in his utility belt and another is located in his left arm. It appears as a virtual computer that he can contact other heroes or find out strategic areas in certain places.
First Aid Kit: Used to aid in treating wounds. Alfred normally uses this to tend to his master's wounds.
Forensics Kit: Batman keeps two of these, one in his utility belt and the other in his vehicles.
Kryptonite Ring: Inside a lead-lined box is a ring with a green kryptonite jewel to battle Superman if he ever goes rogue.