Dave's Imaginary Sound Space has an incredible compendium of links on the history of speech synthesis, complete with sound files, all the way back 50 years to the Sonovox, connected to a Masters' Thesis by Sami Lemmety.

Inspired by Dave's links, CDM has decided to bring you more of the time-sucking power of fascinating speech synthesis links!

Len Sasso has a great article from the 2/2004 Electronic Musician with everything you wanted to know about speech synthesis — history, how to do it, and more copious links!

You can hear how the National Weather Service (USA) has changed its voices
— for those of you who regularly listen to these broadcasts, you can
wax nostalgic about your old friend Paul, who just wasn't as
cutting-edge as the new-fangled Craig and Donna.

And lastly, for the best-sounding speech synthesis I've heard, listen to the eerily-accurate sounds of AT&T's TTS
multi-lingual, interactive demo. Enter text, hear it, and even save as
a WAV. (I want to hear the songs you create with it, okay?) And yes,
you can still do what Radiohead and others have done and use the Mac's speech synthesis capabilities.