In Mac OS X, the say command will take text from stdin or a file and output it to the speakers, an AIFF file, or a service or IP via AUNetSend.
For a quick example, just type say in the terminal, and then each line you type will be said aloud.
Check the man page for more info.
That's it... I just thought this was cool. Feel free to add anything useful.
Inspired by this xkcd comic: