It's easy to make a chrome extension. The first thing you need is a manifest, describing the extension and defining permissions.
As far as I can tell, this is basically the minimum that will work. The manifest version, name, description, and version are all required. Next, you need either a browser action or a page action. A browser action is one that is always available. When you click on the extension's icon, a popup appears, displaying popup.html. Finally, the permissions array lists what the extension is allowed to do. In this case, I gave it permission to ask the browser for information about the active tab.Here's popup.html: All it does is define a div and load a script. Here is popup.js: All this does is asynchronously ask query the browser, filtering for active tab(s). The callback function takes a list of tabs. In this case, the list has only one element. It then finds the tag created in popup.html and sets the content to the active tab url. When you're debugging extensions, you can get access to the regular chrome developer tools for the popup by right clicking on the icon and clicking "inspect popup".