While working on the download site wiki recently I went to the recaptcha website to find only the only wiki listed with a recaptcha plugin was mediawiki. Since I make use of dokuwiki this simply would not do.
So I searched for an existing captcha plugin for dokuwiki to modify and here are the fruits of my labour, a dokuwiki plugin with support for recaptcha. At the moment it lacks foreign translations, but it works, and its running on the download site file wiki.
Simply install the plugin in your wiki, fill in your public and private API keys from the recaptcha website and pick recaptcha as the captcha type.
Credits go to Andreas Gohr for creating the original plugin this is based off of. This is released under GPL V2 in kind.