First announced in March, then found in developer previews, one of the little-heralded new features of OS X Lion is its inclusion of several high-quality text-to-speech voices in 22 different languages. The last major addition to Apple's built-in OS X voices was Alex, a higher-quality voice included in Mac OS X Leopard back in 2007. While Alex was a breakthrough for text-to-speech Mac voices at the time, the over 50 new voices included in Lion outmatch him in several key ways.
These new voices, sourced from Nuance, are not only available in several dialects of English but also, in an OS X first, in several other languages. Text-to-speech voices are now available in Arabic, three different Chinese dialects, Czech, Danish, two varieties of Dutch, Finnish, two French dialects, German, Greek, Hindi, Hungarian, Indonesian, Italian, Japanese, Korean, Norwegian, Polish, two Portuguese dialects, Romanian, Russian, Slovak, two Spanish dialects, Swedish, Thai, and Turkish.
Like a few other features of OS X Lion, Apple hasn't made these new voices easily discoverable unless you know where to look for them. It's also a bit of a misnomer to say they're "included" with OS X, as they are not included in the standard Lion install and require a separate download.
In the Speech pane of System Preferences, clicking on the Text to Speech tab gives you an option for "System Voice" in a pulldown menu. This will likely be set to "Alex" by default. Clicking on "Customize" gives you access to the plethora of new optional voices, and you can play previews of each one before downloading them. (You can also listen to previews of these voices at NextUp.) Most of these new voices sound astonishingly natural, especially compared to the old, robotic, pre-Alex voices that were the bread and butter of text-to-speech in OS X's distant past. In particular, the Australian English "Lee" voice (now my default) and Mexican Spanish "Javier" sound incredibly lifelike to my ears.
Selecting a checkbox next to a voice and clicking "OK" will present an alert asking if you're sure you want to download the voice. You'll find this alert welcome, because these high-quality voice files are huge, generally in the neighborhood of 350 to 500 MB each. If your bandwidth or hard drive space are limited, I wouldn't recommend downloading more than a few of these voices.
I've generally shied away from utilizing OS X's text-to-speech functions in the past, because even "Alex" sounded jarringly artificial to me. The new voices aren't perfect and don't fill every dialectical niche (Richard Gaywood was dismayed there was no "Welsh English" voice, and I'm having to make do with Australian Lee rather than a full-fledged "Kiwi English" voice). That said, many of the new voices sound natural enough that having my Mac "talk" to me is now a useful feature, even though I don't have any accessibility requirements that make them necessary as they are for some users. In particular, Australian voice "Lee" makes my MacBook Pro sound like a bloke worth taking down to the pub for a pint, and that's a feature definitely worth having.