Another well-hidden great secret! Macs come with speech recognition and speech output software. Known as "speakable items", you enable it with "system preferences" -> speech -> "Speakable Items ON".
That pops up a round window with a greyed out microphone. To talk to the computer hold the escape key down. This is known as the "listening key". Release it when you have finished talking. Note: this disables the use of the escape key in all other apps, such as "vi". To regain use of the escape key, turn off speakable items in system preferences. There's also a way to re-map the listening key to be another key, such as F8.
A good question to ask (while holding down the esc key) is "what time is it?" or "tell me a joke".
You can add speakable items to any app. See this Apple article.