Ask HN: Is there any text-to-speech API or library for web apps?

Is there any text-to-speech API or library? I am developing a web mashup and looking for text-to-speech conversion. The content of the site changes, so I can't depend on a virtual character with pre-set text. And the app I am developing is going to be hosted on Google appengine, so if there's any server-side stuff involved, then it needs to be python.