This is an outline of the currency system that was in place when I began (2001) and ran until 2003, a good long time for Agora. It was active and the source of much game play at the time. It was installed before my time, but my understanding is that credit should go to Steve for the major points of this system.
The basis of the system was a currency called Stems (from Stem cells). Every player got a basic salary in Stems, and Officers got a higher salary. But the thing about Stems were, they were very limited in use. You couldn't transfer them to anyone else, or buy much with them. They just accumulated.
Instead there were three currencies that were useful: Papyrii A single Papyrus made a Proposal distributable. Indulgences Penalties for breaking rules or other judicially-bad things were measured in Blots. Blots gave you game penalties (lowered votes, kept you from winning). An Indulgence would destroy a Blot. Voting Entitlements (VEs) Hold a VE, get an extra proposal vote. Max voting power 5.
How did you get these currencies? Each month, an Auction would be held, auctioning off a certain number of each currency. The auction currency was Stems, that was really all you could use them for.
Furthermore, each player had a Role. Scribes (papyrus), Acolytes (Indulgences), and Politicians (VEs). Only players holding the correct role could bid on the correct currency. And roles could only be changed once a Quarter, so you had to plan ahead.
Additionally, each currency was tied to an Office: the Promotor for papyrus, the Clerk of the Courts (Arbitor) for Indulgences, and the Assessor for VEs. Each Officer could control their own currency supply by deciding (in a fixed range) how much to auction each month, and also had the ability to tax (collect a % of everyone's holdings for that currency). Total circulating currency was fixed; direct creation of currency was tightly-controlled and rare, mostly the currencies circulated between players and a Bank. Power over currency supply made these offices desirable, and elections were actually fought based on monetary policies.
Just by the "nature" of play, each currency had a different liquidity.
Papyrus were the bread and butter of activities (in addition to making proposals distributable, you could use them for other parliamentary procedures such as Chambers). Winning at this time was by Points (the original Nomic system), and most of the ways of scoring were voting-related: the old Nomic Prisoner's Dilemma or other scoring rules, which made proposal-manipulation part of the gameplay.
Total VE supply was pegged to the # of players in the game, and permanent votes were powerful. So these became the real estate in the game; precious, commanding high prices at auctions, rarely changing hands.
Indulgences were volatile. The judicial system allowed players to "ticket" each other for minor infractions just by announcement, only needing CFJs if the facts were contested. A day late on a report? Someone gives you a Blot. Sometimes, when players conspired for scams or political plays, all would break the same rule and many would get Blots. Then Indulgences would be priceless. Other times, they were nearly worthless.
Some auctions were snoozers. Some were hotly contested. Various features made for occasionally very hotly-contested events (if a player left the game while still having currencies, all their holdings would be auctioned off as a single lot - very very valuable).
What made this so successful? Part of it was situational; for various reasons, there were some very hotly debated topics (and long-term rivalries) that made proposal and vote struggles very tense and contested. Second, there were a good 20+ active players, a high point (if you look at "departures in 2001" in the Registrar's report, you see many departures - this was the decay of the 2000-2001 boom).
Part is of course my own nostalgia.
But I think there were two parts that made the system itself work well.
First, this system hit a real sweet spot in decision and contestable "exciting" points. Grind with Stems, make strategic decisions with Roles, play them out with the other currencies. Offices with "power" to set policies that had gameplay impacts. Winning was related, but not the whole purpose of currencies. There was enough variety that each "exciting moment" had a different quality and tactics, but not so many options that we were lost in the weeds.
Secondly, as noted by Ørjan, "the three-currency system was also carefully thought out for the purpose of encouraging an economy with meaningful trading - something which we hadn't really managed before. This required making it impossible for a player to alone produce everything e needed, thus both the roles and the three different kind of assets."
Finally, I think we had a period of strong stability in the underlying Rules. At the time, we had players both from the computing world, but also a lawyer or two. The Rules interpretations/CFJ tended to be very "practical" (following intent) rather than letting a twisted wording loophole break things (though some true "infinite loops" and other twisty ideas got through and were respected). The stability was such that a whole family of concepts - Debt, Contracts, Bonds - were made privately (and sometimes added with rules) and some holdings built up over the full 2-3 years that the system lasted.
All in all, it was a Pretty Good Time in Agora.