Here is an example picture demonstrating the Google Homepage edited by me to say that I own Google.
This is just an example. You can use it for anything like showing wrong traffic stats or for fooling people by showing that someone said something about them on Facebook or Twitter. It depends only on your creativity. But I would advise you to stay ethical and use it for entertainment purposes only.
To use this trick, all you need to do is just copy the code given below and paste it in your address bar after you have opened the website you wish to edit. And start editing.
Or, alternatively you can use the following bookmarklets by dragging them to your Bookmarks bar.
The editing that you do will however be temporary as it is not server-sided. You can also save the modified website through your browser by File>Save. This works on all common web browsers including Chrome, Firefox, Opera and Internet Explorer.