Trade exchanges, IIRC. Which I do.
A very simple idea - barter, but with bookkeeping is what it boils down to. Pretty much anyone with a PC (and power, might be an issue going forward for some of us) can create one. If you don't have a PC you can use paper and pen...old skool I know but due to make a come back.
Add to this credit notation for excess and there is a highly viable mechanism for trading between people. John can swap 1 sheep for Daves 1 cow and get a credit of 2 legs of lamb (or something similar) to be repaid later by Dave. These two legs of lamb would then be a credit note which culd be swapped inside the exchange with anyone who thought Dave was good for it - with the usual financial incentives attached - maybe buying a credit note for two legs of lamb in the future would cost 1 and a half legs of lamb right now to the right person, or maybe more if the future for sheep doesn't look all that rosy.
The only important things are -
1) You need something to trade - your time, expertise, goods, whatnot
2) Reliable people to run the admin.
Point 2 is probably what stops trade exhanges from being the totality of trading in the world, to be honest. (Aside from hired goons and the paper moneymen.) Beyond a certain point the risk of not being paid or the likelihood of finding unethical people increases beyond the bounds of system integrity. (Whatever that might mean......sounds good though, doesn't it?)
Typically the admin runners would charge a commision or fee in order to make the trading possible, but nothing too onerous because everyone is replaceable, right?
The other thing you can do is issue a credit card backed by your own goods and services, in exchange for others goods and services right now. The miracle of debt, right?
Have some links -
Just think, you too can have your own currency like the Injindollar.