The Mac OS has long boasted speech synthesis - the capability to read text aloud - but honestly, the quality of the voices, though perhaps better than the competition, is still pretty awful. No one with a choice would listen to Victoria's robotic intonations all day long, although Jeff Carlson's interview with Fred was a hoot (see "," 2003-04-01). (For Leopard, Apple is  - Alex - that's supposed to be much better than the current built-in voices.)
For a far better listening experience that you can try today and that reflect worldwide speech patterns, check out the speech quality of the voices in. They're not perfect; you won't mistake them for real people speaking, but they're a big improvement over the voices Apple ships in Tiger. Designed by the Acapela Group and distributed by AssistiveWare, Infovox iVox provides a wide variety of male and female voices for American English, British English, French, Canadian French, Spanish, American Spanish, Portuguese, Brazilian Portuguese, German, Dutch, Flemish, Norwegian, Swedish, Danish, and Finnish.
I particularly like the high-quality British English voices, perhaps because the slight British accent marks the voice as foreign to my ears, thus causing me to forgive any mispronunciations or hesitations, or rather, to assume that they're part of the accent. And although I'm not sufficiently conversant in any of the other supported languages to understand what's being said, some of them sound even better because of this.
You can on the AssistiveWare Web site, and you can also  to try for a limited time on your Mac. Beware that the voice packs are huge downloads (200 to 600 MB each). The voices work with any application that's compatible with the Apple Speech Manager; Mac OS X 10.3.9 is required as a minimum, but Mac OS X 10.4 is recommended. After running the installer, you can choose different voices in the Speech pane of System Preferences; to test with your own text, look for controls in the Edit menu's Speech submenu (you can always use TextEdit if your preferred writing tool doesn't offer the Speech menu). Through 31-Oct-07, the American and British English voices cost $99 (normally $149 and $219), the non-Scandinavian voices cost $149 (normally $219), and the Scandinavian languages cost $269 (normally $359, but you get the  for free... just kidding).
Needless to say, anyone who relies on the Mac's text-to-speech capabilities will appreciate the Infovox iVox voices (AssistiveWare provides a video showing how a blind translator uses the product), but I wonder if higher quality voice might cause text-to-speech to become more commonly used by those who haven't previously considered the feature before.