Google has updated its Text-To-Speech standalone app with high-quality English voices and new language support. The English US and UK languages now feature high-definition male and female voice packs with smoother enunciation and more natural intonation. Just don't go on talking with your extra-articulate smartphone if there are bystanders around, or you might end up on YouTube. Also, the English US voice has only a high-quality female pack.
The language support has been enriched with the addition of Brazillian Portuguese and US Spanish. Text-to-Speech now supports nine languages in total, but only English has high-definition voices for now. The high-quality voices can be chosen in Android's Settings menu. Go to Language & Input -> Text-to-speech output and select one of the new language packs. You will have to sit through a sizable download to enjoy the HD voice packs, however - they both weigh over 200MB each, while the normal quality recordings are only 6.8MB.
Also, be aware that not all apps follow the setting you choose. Some apps, such as Google Play Books, default to the regular quality voice packs for some reason. Google Translate is keeping up with the times, though, so use that to hear the new voices in their ominous “machine talks like human” action.
In addition, there's a blink-and-you'll-miss-it improvement in the app's user interface. Each voice that has multiple sample packs installed receives a sub-menu with the option to toggle between them. Also, the voice pack sizes are now visible, and the delete button has been replaced with a suggestive thrash can icon.