An improvised weapon is, at its simplest, something used as a weapon that was not designed for that role. Of course that covers a lot of ground.
The simplest improvised weapons are probably things like bits of wood picked up for use as clubs - and that sort of thing is probably the ancestor of melee weapons generally. Where the boundary lies between a bit of wood and a club is open to question1, but even machined pieces like axe or pickaxe handles are fairly effective weapons whilst still being (technically) improvised.
Which brings us on to random manufactured goods, many of which can be pressed into service as (mostly bludgeoning) weapons2 - these are things like candlesticks, chairs and what have you. Broken bottles, pieces of rebar and other assorted pieces fit into this category.
Assorted tools can also be improvised as weapons - most knives are not designed for weapon use, but are quite effective anyway, and the same goes for many axes and, of course, the bill hook, agricultural flail and their like. Others, like the chainsaw, are overrated as weapons. Slightly further afield hammers, saws and other tools can be used if required - the screwdriver being a particularly well known example - and in extremis, even a pen or pencil can inflict significant3 injuries if properly used.
In terms of tools used as weapons, one of the more popular - at least in a military context - may be the entrenching tool. A short, folding spade originally issued to troops for digging with, it often finds itself sharpened and wielded as an improvised close combat weapon as well.
Even more bizarre things have been known to see action - belts remain a popular weapon in street brawls and ties and scarves have long been used to strangle. A substantial weight (like a brick) in a sock or wrapped in a towel can even serve as a rudimentary flail.
More horrific combats may see a severed limb and/or other body parts used as weapons - a sufficiently strong combattant may even use his opponents as weapons.
We've listed improvised weapons as melee, since most of them are, but that's not to say you can't improvise a ranged weapon either - pretty much anything can be thrown to some effect and there are other things - nailguns for example - that can help with ranged fire as well.
Movies: Every Jackie Chan movie ever made
Game and Story Use
- A talent at improvising weapons from pretty much anything is an interesting character trait - whether for an ulimate fighter or just a dangerous psychopath is up to you. Traditionally cinematic slashers have this in spades, but then so do many grittier action heroes.
- Part of the confinement regime for particularly dangerous prisoners is to remove things that they may use as weapons. Where the prisoner has supernatural powers, the range of potential weapons may be greatly extended and may even include some categories of people.