Babble makes your Mac speak to your best friend or your worst enemy. Select a voice in a bunch of languages, type in a text and let it speak. You can even save the spoken content to file or send it as an email.
Mac OS X includes a lot of voices in a plenty of languages: Arabic, English, Czech, Danish, Dutch, Finnish, French, German, Hungarian, Italian, Japanese, Korean, Norwegian, Polish, Portuguese ... On older systems you may have to download these first from within the System Preferences.
Babble is also available on the Mac App Store