This Mac OSX tool uses the Speech Synthesis manager to convert input text to audible speech and either play it through the sound output device chosen in System Preferences or save it to an AIFF file.

# To say a phrase aloud:
say "I like to ride my bike."

# To read a file aloud:
say -f filename.txt

# To say a phrase with a custom voice and speech rate:
say -v voice -r words_per_minute "I'm sorry Dave, I can't let you do that."

# To list the available voices:
say -v ?

# To create an audio file of the spoken text:
say -o filename.aiff "Here's to the Crazy Ones."

# To create an  Audiobook From Text, using "Alex" voice, a plain UTF-8 encoded text file for input and AAC output:
say -v Alex -f file.txt -o "output.m4a"