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
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