The latest revision of the Bitcoin-Qt and Bitcoind clients from Bitcoin.org, v0.5.3, includes bug fixes and support for changes to the Bitcoin protocol.
There was a security update to the Windows versions of the Bitcoin-QT client from Bitcoin.org. Windows users that have a previous 0.5.x release or are testing the upcoming 0.6.x developer release are encouraged to update immediately.
Mac and Linux versions are not affected, nor is the bitcoind command line client impacted. v0.4.x and prior are not impacted either.
See the forum post for more details.
A new version of the Bitcoin-Qt and Bitcoind clients from Bitcoin.org, v0.6.0, includes a number of new features as well as significant performance improvements, bug fixes and support for a change to the Bitcoin protocol.
A new version of the Bitcoin-Qt and Bitcoind clients from Bitcoin.org, v0.6.3, includes a fix for a serious denial-of-service vulnerability and a performance enhancement for new installations.
A significant new release of the Bitcoin-Qt and Bitcoind clients from Bitcoin.org, v0.8.0, is now available for download.
This v0.8 release includes a number of new features, performance improvements and bug fixes and is the first major feature release since the v0.7 release about five months ago. This release was designed to improve performance and to handle the increasing volume of transactions on the network.
A subset of the improvements, features and fixes includes:
With a huge increase in the amount of transaction activity the Bitcoin network is receiving, previous versions of the Bitcoin.org client had been taking an unacceptable amount of time to synchronize the blockchain. This release will reduce the amount of time to reach the synced status to a level that should be acceptable for those with even the least capable computing hardware.
The first time v0.8 is ran the program will migrate the blockchain data and reindex, a process that can take several hours on lower end computing hardware. Proper shutdown from a previous version before installing this release is required.
While it is recommended to back up the wallet.dat before performing this upgrade, the update will not overwrite an existing wallet or blockchain database.
When performing a new installation (versus upgrading from a previous release), a file called bootstrap.dat may obtained as a torrent and placed in the Bitcoin data directory to speed the installation process.
Alternatives to the Bitcoin.org client, now featured on the Bitcoin.org Clients page, have seen updates recently as well:
A review of various wallet options was recently updated by Bitcoin Magazine and is a good resource when choosing a wallet.