These are various options for opening your current URL in nbviewer with one click. They will work if you are looking at any URL that can be pasted into nbviewer.
Any browser can use the bookmarklet. Simply make a new bookmark in your browser and copy the code from nbviewer_bookmarklet.js into the URL field. Activating the bookmark will open a new tab pointing nbviewer at your current URL.
Use the Chrome extension, which adds an nbviewer icon to Chrome's toolbar.
Use the Firefox add-on, which adds an nbviewer icon to Firefox's toolbar.