It is available for Google Chrome on Windows and OS X along with Chrome OS.
Google spoke about ChromeVox:
ChromeVox is a screen reader for Chrome which brings the speed, versatility, and security of Chrome to visually impaired users. Of course since ChromeVox is built by Google, it’s optimized for the web. It supports the latest web standards and excels at accessing the most modern interactive websites.
ChromeVox is an extension for Chrome on Windows and Mac OS X which operates as an alternative screen reader for Web content.
ChromeVox speaks the content of the page, plays audio indicators for page load progress and objects on the page, and provides a way to navigate all web content from the keyboard.