Stop a lot of spambots from polluting your site by making visitors identify a custom word displayed as an image before commenting.
Unzip all files to the folder custom-anti-spam in your plugins directory, so that the path is wp-content/plugins/custom-anti-spam/. Then activate the plugin via your WordPress admin section. The plugin should work directly "out of the box", but all settings can be customized in the Settings > Custom anti-spam page in your WordPress control panel. If you are upgrading from a previous release, de-activate that release first.