Something I ran across recently is the idea of
grain-based local currency - money that is backed by shares of the recent harvest (more details in the link, including some gaming ideas). You can't really save this money for long, since it's only good for the
current harvest of
this village (or town), and once it's either rotted or been eaten, it's gone; it's only redeemable locally, so it's less valuable the farther you are from where it was issued. So, you need to spend it locally, and often on something that'll last longer: better roads, new or repaired bridges, fixing the local church or buying stones and such for the nearest cathedral, building a better mill, and so on. It leads to a different economy than we're used to, where you have little to no money for nine months out of the year, and then a lot of money that must be spent while it's still good.
Obviously, this died out eventually with national currencies and such, but the periods that most 'medieval' fantasy game settings are loosely inspired by include the eras when this would be occurring. In a fantasy setting (at least, one where magic is unlikely to destroy grain-based currencies), some of the long-term items one might pay for are magic items to defend the village from monsters, brigands, and general disasters.
I find myself wondering if anyone has used this in a game, and if so, how it went. Likewise, if anyone is likely to later on.