List of Minecraft Economy Servers, most of Minecraft servers have some kind of Economy going but different servers and gamemodes require different types of Economy.
Currencies on Economy servers?
On Minecraft Economy Servers with classic gamemodes you will generally find Economy based on virtual currency, similar to to real money often called ingame money. Some minigames and gamemoes like Prison would also add additional currencies like Economy tokens and points.
Unlimited vs fixed Economy?
Economy Servers can have fixed or unlimited Economy, on Minecraft servers with fixed Economy there's a fixed amount of money and that's all there is, players have to then trade or work to gain money. With unlimited Economy there is no cap, server could spawn in as much money as needed.
Most of Minecraft servers use Economy plugins as base for currency and Economy itself, plugins like ChestShops, AuctionHouse, Trading Plugin, ... would then simply integrate their features into servers Economy.