Rsynth is a speech synthesis program that runs on Unix-like systems.  It's public domain software as far as I know, because it's based on public domain code according to the README and lack of a license.  It's relatively small compared to other systems (only about 5k LOC).

BSD ports/package systems have rsynth.

Version 2.0 sources can be found here:

http://www.xmission.com/~georgeps/rsynth/rsynth-2.0.tar.gz '''LINK DEAD as at 9 July 05'''

Patches for compiling 2.0 in OpenBSD can be found here (as well as a binary pkg for OpenBSD 3.0):

http://www.xmission.com/~georgeps/rsynth/rsynth-openbsd.tgz
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[Category Speech Synthesis]