Festival is an open source text-to-speech system.
Stanford uses it for their Natural Language Processing class, and they have up-to-date instructions about installation on this cs224s homework page. Installation on Mac OS X requires a couple patches, which they’ve wrapped into a handy install script.
There are alternate voices you can use which sound noticeably better than the stock ones. You can find information on these voices in this forum post:
How to setup more realistic voices in Festival. Those instructions are for Ubuntu, but the voices work with any Festival installation.