Improvised throwing is the use of items which do not train a thrown weapon skill as thrown weapons. However, behavior of these items still differs based on item type.
Improvised throwing weapons[]
This refers to any item that is not a weapon by any definition, but still thrown for whatever reason, such as iron rations or books. These consume 1200 energy points instead of the standard 1000, and damage depends solely on the PC's dexterity (presumably) and the item's inherent damage value. Benefits from Quick Shot, Lightning Shot, or class powers still apply.
Part of this group is a merchant's thrown gold coins. Their damage and to-hit depends chiefly on the merchant's level; they are fully viable thrown weapons. The game has been completed by characters using only thrown gold coins as a means of killing monsters.[1][2]However, they are still truly improvised throwing weapons, requiring 1200 energy cost to use.
Any item category in the game can be thrown, with two exceptions: arrows and quarrels. The game's engine only allows these items to be used in the missile slot in connection with the proper missile weapon.
Weapons[]
(The following observations are based on experiments using a high elven Barbarian with swords and bows.)
Some weapons, such as swords, do not fall under a thrown weapon skill. While this means that they also don't train any, it appears that they do benefit from the corresponding melee/missile weapon skill regarding energy cost, to-hit and damage. Curiously enough, this includes bows; with higher missile weapon skill in bows, damage from throwing the actual bow is also increased. The bow's to-hit and damage properties are also applied. Throwing swords or bows does not, however, train the sword melee weapon skill or the bow missile weapon skill; you have to use these weapons properly to increase in weapon skill. Still, it does appear that if you run out of arrows, quarrels or rocks, throwing your slings, bows and crossbows - for which you should have respectable missile weapon skills after using up all your ammunition - could be a viable last resort. Also, if you happen to be a primary sword user, you might enjoy collecting stacks of short swords and throwing them, backed by your high sword weapon skill.
Presumably, any weapon that doesn't fit a thrown weapon category will benefit from its corresponding melee or missile weapon skill. Apart from swords, that should include whips, staves, two-handed swords (other two-handed weapons, such as heavy clubs, do appear to belong to their proper thrown weapon skills) and possibly maces and flails. Apart from bows, it should also include slings and crossbows.
Note that range remained unaffected throughout all testing, even for thrown bows (which one might expect to see benefited by the missile weapon skill's range modifier). It remained fixed at 2 tiles. So while these weapons might come to deal decent damage, they won't go far.
Shields[]
Shields behave basically like above - thrown shields benefit from the shield skill with regard to to-hit and damage. Presumably to-hit and damage is calculated according to the basic formula for all thrown weapon skills. However, range remains unaffected. Since shields cannot be used as melee weapons - wielding a shield means you attack with the other hand, wielding two shields means no melee attack at all - throwing shields is the only way to do damage with them.
The only - quite significant - difference is that throwing shields does train the shield skill. The game has been successfully completed using thrown shields as the only means of killing monsters.[3]
Range[]
One thing all of the above have in common is that there is no way to increase one's range using them. The range does, however, depend on one other factor: the weight of the item. Clothes, at 40 stones, have a base range of two - ugly clothes, at 50 stones, have a base range of one. Even a zero-weight item doesn't rise above a base range of 2, and there is no value below 1, so it appears that there is only one relevant breakpoint, somewhere between 40 and 50 stones.
A base range of 1 means the player is capable of throwing the item a total distance of two squares; a base range of 2 gives a throwing distance of three squares. This means that at spots where you have to swim only two squares to cross a river, you can throw your low-weight items (such as scrolls or keys) over the river to keep them from being drenched or rusted. You cannot throw quarrels in this manner; you would have to shoot them across instead. If there is a place in which only one water square has to be crossed, you can even get your books over the river safe and dry.
If you cannot throw your items all the way across, it may still be worth throwing them as far as you can if you absolutely have to cross. Items lying in a river do not suffer water damage - they must be in your inventory for that to happen. If you have to cross two water squares, and throw vulnerable items into the second, you can swim there, pick them up and only risk one turn of water damage. This does mean that you will spend one additional turn in the water, however - be careful not to drown!
Unfortunately, if you have to cross at least three uninterrupted squares of water to get over the river, you can only throw items far enough if they receive a bonus from a thrown weapon skill, or, if the PC has the Strong Thrower talent, are merely capable of receiving one.