Lots of ways! First and easiest would be to simply 'inspect popup' on the icon, and then navigate to the sources tab, though you may not get all of the code this way (perhaps if it was conditionally/dynamically loaded).

Second way, and probably the best, would be to go to its store page, and in the URL will be the extension ID; for Ghostery, it's "mlomiejdfkolichcflejclcbmpeaniij". Now, just find that in your Chrome installation directory, e.g. for Win7 the extension would be located at %LOCALAPPDATA%\Google\Chrome\User Data\Default\Extensions\mlomiejdfkolichcflejclcbmpeaniij\.

The last way would be to intercept or otherwise directly down the .crx file (curl, wget, etc.), which is just a compressed zip archive. You can do the same for FF extensions as well, those .xpi files are just zip files.