Typically we copy contents from wikipedia articles – paste it into word documents or other wysiwyg editors. Then format it and print. However after posting the question on facebook questions (http://www.facebook.com/q/Is-there-a-easy-way-to-print-wikipedia-articles-without-the-sidebars-and-see-also-references-etc/10150286691460015?t=1&keep_objects=1), I found that there is an option right there in the sidebar to get printable version or even download it as pdf.
http://techcrunch.com/2009/02/27/print-you-favorite-wikis-as-books-courtesy-of-wikipedia-and-pediapress/ has some more details on new initiatives. We can add multiple chapters into http://en.wikipedia.org/wiki/Special:Book and print out combined articles too.