This page will contain useful information about the tools used in mainstream ICT to collect, organize, refine, and implement user preferences.
Operating System Preferences
Windows has an "Ease of Access Center" (EoA) that lets users control Magnifier, On-Screen Keyboard, High Contrast, and Narrator (the Windows minimal screen reader), as well as other accessibility options. (Vista-era video). Users can work through a wizard ("Get recommendations", which walks through 5 screens of disability categories, asking questions) or directly select the features they want. They can explore settings and have them reset with a restart, or configure them to be permanent.
Note that the main EoA selection screen (in Windows 8) contains the following text: "Your answers are used to determine recommended settings only. If another program or Web site wants to use this information to better suit your needs, you will be explicitly asked for permission by that program." This is followed by a link to the Windows privacy statement.
Application-specific settings. Some applications contain accessibility-relevant settings
Control by choice of application. In some cases, the user can use a different application altogether; one which has been customized with specific accessibility features, or is more accessible by coincidence.
IE, Firefox, and Chrome have zoom controls. In Firefox these apply per-tab. Very discoverable and easy to use.
Extensions and other add-ons give users another way to control browsers:
Screen readers ChromeVox, Voxzilla
Other text-to-speech tools Google Chrome Read&Write Toolbar
Themes (for font and color control)
Extensions are at the other end of discoverability and usability: the user must know which one he/she wants, find it in a store, install it, and sometimes re-start the browser. Interactions with other extensions are generally unmanaged and unpredictable.
Websites and web apps
Some sites show text-size widgets. Some CMSs and web development tools support this feature with plugins. Not sure if there is any persistence of the user's choice for subsequent visits.
Sicap Self Help
A tool aimed at wireless providers so that their support agents can help end users configure the devices for advanced services, including images and step-by-step instructions, or that the providers can put that information on their websites where end users can find and use it. Contains interesting data: 23% of devices can't be configured over the air (OTA), so users call support agents. ~2/3 of users don't use advanced features. 40% try to find wireless answers online.