I have a relative suffering with this condition. They are often unable to enunciate words and their hands and arms shake during more extreme episodes.
I remember reading about software such as Dasher, which is available for Ubuntu. Are there any other applications that Ask Ubuntu members have found useful for sufferers of Parkinson's disease? The goal would be to help them communicate.
Edit: The goal is to help them communicate with people around them rather than help them do tasks on the computer.