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. A shared library version is also available.
- Includes different Voices, whose characteristics can be altered.
- Can produce speech output as a WAV file.
- SSML (Speech Synthesis Markup Language) is supported (not complete), and also HTML.
- Compact size. The program and its data, including many languages, totals about 1 Mbytes.
- Can translate text to phoneme codes, so it could be adapted as a front end for another speech synthesis engine.
- Potential for other languages. Several are included in varying stages of progress. Help from native speakers for these or other languages is welcomed.
- Development tools available for producing and tuning phoneme data.