AT&T Natural Voice
NeoSpeech
Acapela
Realspeak
Loquendo
Cepstral
IBM TTS
tkexpert
I like the idea, I ran into some voices that were missing and Sapi 4 was changed to Sapi 5. After downloading the voice I had used in the past I was happy but had to change back to Sapi 4. Infomation can be found on the web, quite a few free voices but many can be expensive making it hard to evaluate them.
Carl2
Though this was done some time ago, it might shed some light.
Scroll down to my critique of various TTS voices / vendors.
http://www.zabaware.com/forum/topic.asp?TOPIC_ID=2227&SearchTerms=TTS,voices
Long Live Valley Girl [:D]