Most modern operating systems are capable of text to speech voice synthesizing. This allows you to have your computer read entire webpages or selected text on your screen out loud to you.
For Windows, hold the Ctrl key and press the Escape key, then type “narrator” and then press the Enter key.
For Mac OS X, click the Apple menu in the top-left corner of your screen and go to “System Preferences”, then choose “Dictation & Speech”, then click “Text to Speech”. Check the “Speak selected text when the key is pressed” checkbox.
More information about Mac OS X’s text to speech features.
Third party software is also available: