A design document shows an example of Google Chrome extension. For now, extensions are just fancy wrappers for user scripts, but there are plans to make them more useful by exposing browser features and allowing developers to create interfaces.
Before trying to install the extension linked at the bottom of Google's tutorial, you need to have a recent developer build or Google Chrome 2.0 beta.
To enable the support for extensions, edit the target field of the shortcut you use to launch Google Chrome. Right-click on the shortcut, select "Properties" and append to the Target field a space followed by:
Download the extension, extract the content to c:\myextension and launch Google Chrome. The only visible effect of the extension is that it replaces the logo from Google's homepage with a lolcat, but you can edit the file foo.js and enter a different URL for the image.