eSpeak is a compact open source software speech synthesizer for English and other languages. eSpeak uses a formant synthesis method. This allows many languages to be provided in a small size. It supports SAPI5 version for Windows, so it can be used with screen-readers and other programs that support the Windows SAPI5 interface. It can translate text into phoneme codes, so it could be adapted as a front end for another speech synthesis engine.
Festival offers a general framework for building speech synthesis systems as well as including examples of various modules. It offers full text to speech through a APIs via shell and though a Scheme command interpreter. It has native support for Apple OS. It supports English and Spanish languages.
Linux/Ubuntu - Speech Synthesis from Java without …
All Cepstral voices come with the powerful and robust Swift Text-to-Speech engine - use it from the command line to synthesize text or files to an audio device. Or save the audio to a file so you can listen to it later.