The TTS engine built in to OSX works offline and is good. In my opinion it's as good as any of the online ones for most uses. But the OSX license limits it to

The TTS engine built in to OSX works offline and is good. In my opinion it's as good as any of the online ones for most uses. But the OSX license limits it to "personal, non-commercial use". I'm not sure whether it's possible to get the same technology in a commercial license. Some of the voices are licensed from Nuance (parent company of Dragon), but it's not clear whether the engine is too. Some better free-software offerings would really open this area up for experimentation, since the commercial offerings tend to be pretty black-box and targeted at very specific use-cases.