I got a lot of spam from the Joomla contact form, I need a captcha plug-in. Actually, there are a lot of anti-spam plug-in in Joomla. But they are fairly complex, some of them even have a custom-build contact form. That is too much.
After I spent some time to research on the internet, I found a plug-in calls Joo ReCaptcha. That is very simple. After I installed that, the contact form will have ReCaptcha image under the input fields. That is great!