Bitcoin-Qt

Bitcoin-Qt is the third Bitcoin client, developed by Wladimir J. van der Laan based on the original reference code by Satoshi Nakamoto. It has been bundled with bitcoind since version 0.5.

History
Bitcoin-Qt can used as a desktop client for regular payments or as a server utility for merchants and other payment services.

Current version
Source code (and build instructions for supported platforms) can be found at the Bitcoin GitHub page.

Features

 * Compatibility with Linux (both GNOME and KDE), Mac OS X and Windows
 * All functionality of the original wxWidgets client
 * Asks for confirmation before sending coins
 * CSV export of transactions
 * Clearer transaction list with status icons and real-time filtering
 * Progress bar on initial block download
 * Languages: Dutch, English, German, many more
 * Sendmany support in UI (send to multiple recipients in one transaction)
 * Multiple unit support, can show subdivided bitcoins (mBTC, µBTC) for users that like large numbers (only decimal units)
 * Splash screen that details progress

Criticism
Bitcoin-Qt is often criticized for being slow in downloading and verifying the Bitcoin transaction database (often called the Blockchain).

Wallet management is also cumbersome. Unlike clients such as MultiBit, only one wallet at a time is supported, and its location is required to be the same as the blockchain storage, making it difficult to place the wallet on an encrypted drive.