To use PrintWhatYouLike, copy the url (the http://www… address) of a page, paste it into the ‘Enter a url to start’ box, and hit start. A new page will appear containing your original page, plus a toolbar. With the toolbar you can reformat the page until it looks good enough to be print. When you do click ‘print’, only your modified page will actually appear on the paper.
There is a great screencast of the printwhatyoulike.com in action here: http://demogirl.com/2008/09/24/print-only-the-content-you-want-with-printwhatyoulike/