Bitcoin is an experimental new digital currency that enables instant payments to anyone, anywhere in the world. Bitcoin uses peer-to-peer technology to operate with no central authority: managing transactions and issuing money are carried out collectively by the network. Bitcoin is also the name of the open source software which enables the use of this currency.
The software is a community-driven open source project, released under the MIT license.
Latest version: 0.5.2
Windows (zip) ~8MB
Windows (exe) ~8MB
Ubuntu PPA
Linux (tgz, 32/64-bit) ~12MB
Mac OS X ~6MB
Press mailing list for presentation and interview requests: 
9 January 2012
Bitcoin version 0.5.2 is now available for download at: http://sourceforge.net/projects/bitcoin/files/Bitcoin/bitcoin-0.5.2/
This is a bugfix-only release.
Please report bugs using the issue tracker at github: https://github.com/bitcoin/bitcoin/issues
Check all transactions in blocks after the last checkpoint (0.5.0 and 0.5.1 skipped checking ECDSA signatures during initial blockchain download; this was not a security vulnerability).
Cease locking memory used by non-sensitive information (this caused a huge performance hit on some platforms, especially noticable during initial blockchain download).
Fixed some address-handling deadlocks (client freezes).
No longer accept inbound connections over the internet when Bitcoin is being used with Tor (identity leak).
Re-enable SSL support for the JSON-RPC interface (it was unintentionally disabled for the 0.5.0 and 0.5.1 release Linux binaries).
Use the correct base transaction fee of 0.0005 BTC for accepting transactions into mined blocks (since 0.4.0, it was incorrectly accepting 0.0001 BTC which was only meant to be relayed).
Don’t show “IP” for transactions which are not necessarily IP transactions.
Add new DNS seeds (maintained by Pieter Wuille and Luke Dashjr).