Via Aran, of Organic Design New Zealand:
More info via http://www.bitcoin.org/
Bitcoin is an open source peer-to-peer (a.k.a "p2p") electronic cash system that's completely decentralised, with no central server, trusted authorities or middle men. The availability of bitcoins can't be manipulated by governments or financial institutions. Bitcoin already has a number of exchanges for converting to and from other currencies; BitcoinFX, New Liberty Standard, Bitcoin Exchange and Bitcoin Market.
Bitcoin may last for years and become a popular global currency, or it could be just a flash in the pan, but either way I think this is an important sign of the times to come. This is one of the first truly decentralised currencies and has paved the way for hundreds more to compete together in the new arena of Cipherspace over the coming years. This is one of the key factors in the transition of global society into the post-nation-state economy talked about in The Sovereign Individual.
In a p2p computer network there are no servers, the entire network is composed of users running instances of the application on their computers. Each running instance offers a small amount of processing and storage resource to the network so that it can deliver the services it was designed for such as redundant storage, anonymity or voice-over-IP applications.
In the case of a p2p currency system, some of the services the network is designed to offer are privacy, verification, authentication, currency creation and transfer of ownership. To ensure a reliable and tamper-proof system requires a lot of resource, and that amount is proportional to the amount of coins in the network. The network is able to pay the users for the resource they offer by making the coin-creation process part of the network protocol itself instead of being handled by a central trusted authority. This creates a natural and incorruptible link between the supply of currency in the network and the demand for it.
Even aside from the ability to exchange bitcoins for other currencies, it still makes a very useful tool for independent organisations and groups because it allows them to trade and settle accounts amongst themselves independently and privately. It effectively gives them a "bank" that has a trustworthy system of accounts that can't be tampered with and requires no corruptible central authority to operate. See the Bitcoin Whitepaper for more detail about how it works.
To try Bitcoin, download the Bitcoin software, then once it's running, click 'Generate Coins' which will pay you bitcoins in exchange for your computer working to validate bitcoin transactions. Check the exchange rate to calculate how many bitcoins need to be sent. The payer can purchase additional bitcoins if needed. The payer's previously generated bitcoins allow for a lower out of pocket payment. The payer then sends the bitcoins to the receiver using the Bitcoin software. The receiver can then sell their bitcoins for dollars. The receiver's previously generated bitcoins allow a higher dollar payout.