Starting with version 17, the Google Chrome browser ships with support for modifying the User-Agent string. (Click here for the Chromium issue and here for more info on the feature.) This renders the dedicated user-agent switching extension obsolete, but the approach outlined here may still be relevant in the context of a larger extension.

Allows you to re-set the "User-Agent" string in Chrome's HTTP request headers.

Download the Extension. You can get the latest version of the extension here (of course, you'll need Google Chrome installed first).

Install the Application. Drag the extension file (it is named 'chrome-ua-modifier.crx' by default) into a Chrome browser window.

Changing Your User-Agent. The extension options can be accessed either by selecting "Options" under the extension's entry on chrome://extensions or clicking on the extension's logo () next to Chrome's omnibox. Select any entry from the device list, and all future HTTP requests will be sent with the user-agent string displayed in the text area. Select "default" to use your default user-agent string.

Adding/Modifying "Devices". Click "Add a Device" to create a new device--be sure to type a device name in the text area that appears. (You can double-click any device to change its name.) If you want to change the User-Agent string associated with any device, select it from the list and modify the text in the "Current User-Agent" text area. (I have found this page on zytrax.com to be a great resource for valid user-agent strings.)

Deleting "Devices". If you no longer need a saved device, simply double-click on its entry in the list and clear the text area.