MBROLA:
the MBROLA project based at Faculté Polytechnique de Mons. This
project offers diphone synthesizer for many languages, which can be
used as the basis for new full TTS voices. (local US mirror
is available here)
OGI TTS Group
Oregon Graduate Institute offer new voices in Festival (English,
Spanish and German).
The EMU Speech
Database System offers speech display and label editing software
useful in building, and diagnosis faults in synthetic voices.
(local US mirror is available here)