espeak-ng

Software speech synthesizer

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.