All you have to do is open Applications > Utilities > Terminal. Once the Terminal window opens type Say and hit return (then repeat process).
Then you can go to System Preference and select Speech to control speaking rate and choose from different voices like Fred, Kathy, Princess, Cellos, Boing, Zarvox, Bubbles, Bad News and so on… Post in the comments any creative uses you find for this feature. You can copy/paste large sections of text or song lyrics and have your computer read your children to sleep… or is that unethical? Why not leave a message on your friends answering service in robot?
Now here’s a creepy video of a kid you can be very thankful was never your boyfriend in high school: