In the Universal Access Folder (formerly named Easy Access), you can find out about the following:

Note: this information is taken from Apple Accessibility Features Page

Mouse Keys
MouseKeys is a program that lets you control all mouse movements by typing on the numeric keypad. This utility is especially valuable for people who lack the manual dexterity to maneuver a mouse.

Sticky Keys
StickyKeys is a software keylatch, meaning it can help you hold down keys during times when you must press two or three keys simultaneously (if you need to type a capital letter or a question mark, for example). For people with a physical disability who type with one finger or with a mouth wand, this keylatch feature is a helpful tool.

Slow Keys
SlowKeys enables the user to change the length of time it takes for a keystroke to be registered on the screen. This allows for several keys to be pressed accidentally without effect. For a child or adult with fine motor difficulties, this customizable feature saves unwanted keystrokes from appearing on the screen

Text-to-Speech
Your computer can speak the alert messages that appear on the screen. 26 different voices to choose from. Set your computer to speak alert messages using the Talking Alerts section of the Speech control panel. In some application programs, your computer can also read text contained in documents out loud

CloseView
CloseView is a screen enlarger. It magnifies all screen images (including text, graphics, menu bar, and the mouse cursor) up to 16 times the normal size. In addition, CloseView can inverse the Macintosh display (so that text appears white on a black background instead of vice-versa). Naturally, these features are used most often by individuals with visual impairments.

PlainTalk:
Apple's Voice Recognition software. The latest version of the Apple OS will have this feature or you can download them from the Apple Site.
Go to Apple's Speech Recognition Page for directions.