Path of Exile's innovation continues with the game's currency system. It is entirely possible that other games have used a similar system but personally I don't remember playing any RPG which didn't have a form of fiat currency such as gold or credits.
Path of Exile uses a variety of consumable items instead of currency. Since all magic items (and up) are unidentified when they drop, an identify scroll is required. Thus, Scrolls of Wisdom. These are one of the first items of currency that you will find in the game. In fact, your character starts with a couple in their inventory.
At the beginning, these scrolls are fairly rare drops and you'll find yourself incapable of identifying every magic item that drops. You will soon learn to simply sell most items without identifying them and save your scrolls for those items which might be good for you.
Apart from identifying items, Scrolls of Wisdom are also used to purchase certain items from vendors. You will also receive scraps from these scrolls by selling items to vendors. Once a certain number of scraps stack, they convert automatically into a Scroll of Wisdom. Similarly, there are other items of currency. Notice the example below. I sold the piece or armor, wand and flask for 2x Alteration Shards and 2x Transmutation Shards. Each of them stack to 20, meaning that once I have accumulated 20 of each, they will convert into an Orb of Alteration and an Orb of Transmutation, respectively.
As shown above, there are a lot more currency items which drop, or which can be combined from "scraps" and "shards" that are obtained from vendoring items. Currently I have discovered 14 other such items. To give some examples, there are Armorer's Scraps (which can be applied to armor for additional armor value), Orbs of Transmutation (which can turn a normal item into a blue, magic item), Orbs of Chance (which upgrade a magic item's quality randomly, and this is a cool one because you can convert a blue item into a yellow one), Orbs of Alchemy (which turn white items into yellow ones), Orbs of Regret (which allow you to recover a passive skill point previously assigned) and so on. There are items which re-roll the stats (prefixes and suffixes) on an item, or those which allow you to re-roll the sockets on items, or those which allow you to re-roll the color of those sockets.
From what I can tell, this system's biggest advantage should be that it keeps inflation in check. The in-game currency is also a consumable. Whether you like it or not, you're gonna have to use these currency items at some point, if you want to improve your character. This means that the currency is pulled out of the system, without being hoarded. The currency itself is the "gold" sink.
As mentioned, a lot of these Scrolls and Orbs and such drop from monsters so they are quite random. Certain ones are much more scarce while others (like the aforementioned Scrolls of Wisdom) are more abundant.
If you check out a vendor's inventory you'll notice that items have various costs, and an example is shown below, where the weapon can be bought with 1x Orb of Transmutation.
While the system is very unique, there are a few big problems that I can see. It's hard to engage in trade with other people. The chat channels are brimming with items for sale but for the life of me I have no idea how these people trade. I mean, there are at least 15 currencies that you can trade stuff for. What are the chances you'll find someone who has the exact currencies that you need?
The game also doesn't seem to have an auction house but even then, I can't imagine how it would work. I guess you would have a choice of currencies for which to sell your items. In any case, personally I feel intimidated to trade anything. I don't even know at this stage which currency is more valuable in the long run. I just vendor off all my stuff instead of wasting time on the chat channel but this is not at all profitable.
I will probably try my hand at some trading later on because I'm curious as to how exactly this is accomplished. Right now, though, I'm too busy having fun playing the game.