The top free and open-source AI library for implementing text-to-speech functionality is called Whisper. It is a product of Open AI and was released in September 2022. Whisper is an automatic speech recognition model that offers transcription and translation services with high accuracy. It can be a great option for developers who are looking for a powerful and flexible tool to add text-to-speech functionality to their applications. Whisper supports multiple languages and can be integrated with various platforms and frameworks.