A co-worker pointed me to AT&T Natural Voices, a Text-to-Speech research project from AT&T. I tried the online demo and I'm really impressed.
It's been a long while since I last tried a TTS system, so may be I'm just late to the party... but this one produces very natural speech. I tried a couple of English and Spanish voices, and all seem very real.
I knew about the Festival project, which provides a free speech synthesis system, but if you check the online voice demos you will probably agree that it's not on par with AT&T's.