The answer NOW is Alex, included with Leopard and Snow Leopard--it's at least as good as anything Cepstral offered the last time I checked. At the time of the original question back in 2006, though, there wasn't any.
The older stock voices--Bruce and Vicky--are reasonably understandable, though. I say this from a lot of experience, as a throat disorder rendered me unable to speak and relying entirely on a homebrew "talk for me" app I wrote to make noise for me during most of 2006. Bruce got the job done, though Cepstral was nicer had I been willing to fork out for them. I sure would've liked the option of Alex, though.
