For the cognitively challenged
There are some specific efforts to make computers accessible to people with cognitive challenges. Let us focus on an important and widely prevalent challenge— dyslexia. Wikipedia defines dyslexia or the developmental learning disorder as difficulty with learning to read fluently and with accurate comprehension despite normal intelligence.
Dyslexic people would find it difficult to read the text appearing on the screen. There are specialised fonts that make reading simpler for them. One such open source effort is at http://opendyslexic.org/ which provides a free and open source typeface. By simply installing the font or browser extension, you would get this feature in your system. If you are already using Linux distros like Fedora, Arch Linux, etc, this package is available in their default software repositories.
A screenshot of the Open Source For You home page