I remember playing with an ICQ plugin a while back called Merlin, who could read your ICQ messages, or any other text for that matter. The speech synthesis used was rather uninspiring, but fun to play with nonetheless. Searching for speech synthesis, I came across a Wikipedia entry on speech synthesis. Its rather amazing how far speech synthesis has come. And, I must add, its a bit too much fun to play with. Here are some free demos of some commercial speech synthesis programs:
Check the Wikipedia entry for more links.