The WikiPedia page on Batman's utility belt has a decent list of what he may or may not carry around.
The items that are almost guaranteed to be found on every variation are
- Grappling hook
- Lock picks
- A way to communicate (with the Batcave, at least)
As for the grappling hook, sometimes it shoots directly from the belt, and sometimes it's a collapsible launcher that he stores in or on the belt:
Sometimes it's a full-size gun that is simply holstered on his belt. You typically find him using a holstered gun in movies, such as the Nolaverse Grapple Gun:
Like the WikePedia article says, he's likely always carrying something to pick locks, although the picks may be stored somewhere like his gloves, so he can access them even if his hands are cuffed.
His Batarangs also come in many designs, but are generally collapsible and fit within his utility belt or like the Nolanverse suit are small and can be tucked in.
Aside from those three items, his loadout isn't necessarily "standard", but I would also count on something like smoke pellets to create cover, something to create a distraction (like flash pellets/explosive), and some standard detective gear: camera, light source, voice recorder, fingerprinting kit.
Those items are typically found within the utility belt, as well.
For reference, here's some variations of his utility belt (click for full-size):