eSpeak is a compact open source Linux and Windows speech synthesizer for English and other languages. eSpeak uses a “formant synthesis” method. This allows many languages to be provided in a small size. The speech is clear, and can be used at high speeds, but is not as natural or smooth as larger synthesizers which are based on human speech recordings.

eSpeak is available as:

• A command line program (Linux and Windows) to speak text from a file or from stdin.
• A shared library version for use by other programs. (On Windows this is a DLL).
• A SAPI5 version for Windows, so it can be used with screen-readers and other programs that support the Windows SAPI5 interface.
• eSpeak has been ported to other platforms, including Android, Mac OSX and Solaris.

speak.sourceforge.net