The Curtain University Center for Accessible Technology, (CUCAT), has posted a Mac OS X installer package for ESpeak, an open source text-to-speech engine for Macintosh, Windows, and Linux. The CUCAT installer package will install the software for you, and direct you to documentation on the synthesizer. Unfortunately, ESpeak does not integrate with the Mac’s speech system, so it cannot be used with VoiceOver or other Mac OS X speech services. Some applications may include direct support for ESpeak however.

You can download the package from this page at CUCAT.org.