Browsealoud is free downloadable software that will read webpages out loud for you. Also available for Mac OS X.

JAWS is a popular professional screen reading software, and as such is also quite expensive.

Window-Eyes is another professional screen reader which also has support for braille displays.

From the Apple menu: Select System Preferences> Universal Access > Seeing

Here you can activate VoiceOver (also called Spoken Interface on older versions) which will read out text to you through your computer speakers. VoiceOver which comes with Mac OS X 10.4 and above is a fully functional screen reader, just as good as the professional ones you would normally have to pay for.