The original Bitcoin software by Satoshi Nakamoto was released under the MIT license.
Most client software, derived or "from scratch", also use open source licensing.
Bitcoin is the first successful implementation of a distributed crypto-currency, described in part in 1998 by Wei Dai on the cypherpunks mailing list. Building upon the notion that money is any object, or any sort of record, accepted as payment for goods and services and repayment of debts in a given country or socio-economic context, Bitcoin is designed around the idea of using cryptography to control the creation and transfer of money, rather than relying on central authorities.
Bitcoins are sent easily through the Internet, without needing to trust any third party.
Are fast. Funds received are available for spending within minutes.
Cost very little, especially compared to other payment networks.
The supply of bitcoins is regulated by software and the agreement of users of the system and cannot be manipulated by any government, bank, organization or individual. The limited inflation of the Bitcoin system's money supply is distributed evenly (by CPU power) to miners who help secure the network.
In the news
BitPay reports a theft of 5,000 BTC and is suing its insurer for declining to pay a $1 million settlement.