ACAT (Assistive Context-Aware Toolkit) is probably the most famous AAC software in the world as it was created for and with Professor Stephen Hawking.
In addition to AAC, ACAT also provides switch access to most Windows software and includes very capable mouse emulation functions.
The software can be operated using a single physical switch or with a standard webcam and the included ACAT Vision application which allows small facial gestures to be used to control it.
Intel Labs have released the software as Open Source - as well as being free for anyone to download and use, other developers are allowed to build on the work, improve the software or add new features and hardware support.
Note that while most of the software runs on any version of Windows, the ACAT Vision software requires a 64-bit version of Windows.