411 REFERENCE · Wikibooks
CAPTCHA/Integration/Plugins for software
Reference material by Wikibooks contributors. This retains its source identity and is not 411 news reporting.
WordPress
There are many CAPTCHA plugins available for WordPress. Search with Google.
DokuWiki
A DokuWiki plugin is available and has many types of CAPTCHAs available.
MediaWiki
ConfirmEdit is a extension for MediaWiki that provides a CAPTCHA and is configurable. It is well known as Wikimedia Foundation projects use them.