Note Chrome 17 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.
Install the Application. Drag the extension file (it is named 'chrome-ua-modifier.crx' by default) into a Chrome browser window.
How to Run
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.