Google Chrome would soon allow changing user agent from the browser itself in a future edition.
This feature has been added in the dev channel edition and is accessible from the developer’s tools.
Google offers some pre-defined settings targeted at the developers: Internet Explorer 7/8/9, Firefox 4/7 for Windows/Mac, iPhone, iPad and Nexus S running Android 2.3.
You can enter a totally customized user agent as per your wish.
Google Chrome of course already supports such feature using browser extensions and command line parameters. This should make it more convenient.