Google announced the availability of Chrome 21 today, a new version of the increasingly popular Web browser. It includes a number of improvements and several noteworthy new features, including proper support for Apple’s retina display.

Under the hood, Chrome 21 brings official support for the getUserMedia JavaScript API, a feature that allows Web applications to access the user’s webcam and microphone. We first reported on the getUserMedia API back in January when Google originally introduced experimental support in the developer channel.

In that article, we demonstrated how to use the feature to build a simple photo booth Web application. At the time, the feature could only be accessed by launching the developer build with a special flag. In Chrome 21, the feature is now available out of the box.

When a Web application attempts to access a camera or microphone stream, the browser will display a prompt requesting the user’s permission to proceed. The bar that displays the prompt will also allow the user to select which devices to use for audio and video. This makes it possible to, for example, indicate that you want to use a USB headset for audio.

To illuminate the capabilities that getUserMedia can unlock on the Web, Google’s Chrome 21 release announcement highlights several impressive demos, including a virtual xylophone that uses real-time motion tracking through the user’s webcam.

The getUserMedia API is obviously useful by itself, but it’s going to get even more useful in the future. It’s one part of a broader set of emerging standards called WebRTC, which aims to enable real-time audio and video messaging on the Web. It could eventually make it possible for services like Google+ Hangouts to be implemented entirely with standards-based Web technologies, obviating the need for plugins.

In addition to out-of-the-box support for getUserMedia , the new version of Chrome also adds support for retina displays and offers improved support for the gamepad input devices. Google first announced retina compatibility for developer channel builds in June shortly after Apple unveiled the retina-enabled Macbook Pro.

Chrome 21 is available for download from Google’s website and is already being deployed as an update through the stable channel.