The MediaWiki captcha plugin is named the '[[http://meta.wikimedia.org/wiki/ConfirmEdit_extension|ConfirmEdit extension]]'. This causes captcha images to be displayed to help keep out automated spam bots.  See it action at [[http://test.wikipedia.org/wiki/Main_Page|test wikipedia]].  If you try adding an external link, it comes with the message:

"//Your edit includes new URL links; as a protection against automated spam, you'll need to type in the words that appear in this image: //"

See also their [[http://meta.wikimedia.org/wiki/Anti-spam_Features|Anti-spam features page]] for other spam fighting ideas