Since I’m thinking about all things WordPress, I’ve updated my Custom Captcha for WordPress into a proper plugin called Stop Registration Spam.
The idea is the same as before: to prevent automated registrations, and the spam that comes with them, by posing a question that will be simple for your visitors to answer, but impossible for a robot.
It now includes proper configuration via an options page in the WordPress Settings menu, where you can enter a unique question, answer and error message without altering the code.
You can download the plugin from the WordPress Plugin Directory, here.