Can my Mac tell jokes?
Why yes, it can, as a matter of fact. Follow these steps to hear some high quality jokes on your Mac.


Open up Speech Recognition in your system settings. 
Enable "Speakable Items"
Enable "Speak Command Acknowledgement"


Set up the rest however you like (in regards to key press or keyword for input). When you're ready, say "Tell me a joke." (Or, if you have a keyword " tell me a joke".)

Enjoy! (Tested in Lion, although this has been around for quite some time.)