OS X has a neat text-to-speech engine that'll read back what you write. You can start and stop TTS from the contextual menu or launch it using a keystroke that you set up in the Speech section of the System Preferences. Most of the settings for TTS are buried in the System Preferences which is inconvenient when you want to change a setting on the fly. If you need more flexibility than what OS X offers, you should take a look at Text2Speech. Text2Speech is
