Accessibility

It is important to consider accessibility when designing a user oriented product so that it reaches the largest possible audience.

Features

Basic accessibility support, which will be present in any modern operating system without additional software. This includes features like:

• text zoom,
• screen zoom,
• cursor magnification and highlighting,
• on-screen keyboard support,
• text-to-speech for alerts and other critical system events,
• spatial and/or monaural audio,
• and global white-on-black and/or grayscale display modes.

Mac OS X, Windows and most (all?) of the various contemporary Linux distributions support the majority of these features by default.

Source: http://mattgemmell.com/2010/12/19/accessibility-for-iphone-and-ipad-apps/