All held items which cannot be used can be thrown. This can be done to transport an item you don't want to carry, disarm an arrow trap, or discard junk. Additionally, throwing items is one of the basic ways to attack a monster (along with using a weapon, or jumping on top of monsters).
Pots and skulls will break if they hit an enemy or if they hit a block with anything but the lightest of force. Arrows will be consumed if they hit an enemy (in HD they will be turned into near-useless headless arrows). Until they explode, primed bombs behave like other throwable items.
Other items can be thrown repeatedly and can kill an unlimited number of monsters. The only practical use of the stone is as a projectile and many other objects can also be used similarly, for example flares, keys, chests, and corpses.
The distance and speed with which an item can be thrown is dependant on the item's weight, with light objects travelling further and faster than heavy ones. Regardless of weight, all items will do 1 point of damage to enemies upon contact.
Functional objects, such as weapons, can't be thrown in the same manner. However crouching and pressing the action button will release the item and send it slightly forward, which may be enough to send it off a ledge.
A regular throw moves sideways, with a slight arc high enough to hit the ceiling if you're in a gap only one block high. Holding the up direction while throwing will launch the item upward at a 45 degree angle starting from the object's current location in your hands. Holding the down direction will do the opposite while jumping or holding a ladder, but will result in simply droping the item if not. Smaller items will bounce a bit when thrown into the floor, larger items are effectively dropped.