WPMU-Signup-Captcha is a plugin to add a captcha image to your WordPress MU new blog/user registration form in order to prevent spambots from registering in your website. It doesn't require any file modification. Just copy the whole directory structure to the root of your WordPress MU installation. Note that this plugin works with versions r925 and above! The latest version fixes a nasty bug setups with a PHP optimizer enabled. If you experience any problems with the old version try upgrading. Note: The patch installs a plugin in the mu-plugins/ directory. This means the plugin doesn NOT appear in your plugin management screens.