The reference Bitcoin software
Why was the Bitcoin software previously called Bitcoin-Qt?
The original graphic user interface version of Bitcoin, developed by Satoshi Nakamoto, was simply called “bitcoin.exe”. The original release used the wxWidgets C++ libraries to build the user interface. As wxWidgets was not well known and difficult to work with for the current developers, as of version 0.5 the GUI was migrated to the Qt framework. Due to this large feature change, the executable program was renamed “Bitcoin-Qt”, and has become the unofficial name for the GUI version, although this name is not shown in the Bitcoin wallet software.
Because of the obscurity of this name, as of version 0.9.0, the full version of the reference client was given the name Bitcoin Core. The actual program file is still named bitcoin-qt (or bitcoin-qt.exe on Windows).
Current and past builds of Bitcoin:
Bitcoin source code archive