eSpeak is a compact open source software speech synthesizer for English
and other languages.

eSpeak produces good quality English speech. It uses a different
synthesis method from other open source TTS engines, and sounds quite
different. It's perhaps not as natural or "smooth", but I find the
articulation clearer and easier to listen to for long periods.
It can run as a command line program to speak text from a file or
from stdin.

WWW: http://espeak.sourceforge.net/