The MediaWiki captcha plugin is named the 'ConfirmEdit extension'. This causes captcha images to be displayed to help keep out automated spam bots. See it action at 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 Anti-spam features page for other spam fighting ideas