Anti-Spam Features. CleanTalk protects your website from spam bots and spam in Comments, Orders, Widgets, Registrations, Bookings, And any custom web form, Contact Emails, Subscriptions, Checks the existing comments and users for spam.
It allows you to filter and search for spammer comments on your channel and other's channel(s) in many different ways AND delete/report them all at once (see features below).
Akismet checks your comments and contact form submissions against our global database of spam to prevent your site from publishing malicious content. You can review the comment spam it catches on your blog’s “Comments” admin screen.
Mollom is a freemium web service to get rid of spam, unwanted content and fake users on your Drupal, Wordpress and Joomla site or blog.Discontinued
"The Mollom content moderation service is no longer available … as of 2 April 2018."
Anti-spam plugin blocks automatic spam in comments section. No captcha. Plugin is easy to use: just install it and it just works. Blocked comments can be stored in the Spam area if needed. This can be enabled/disabled via Settings page.
A lightweight (~217 bytes vanila JS at frontend) and privacy-friendly plugin to prevent spam comment in the default commenting system of WordPress. Works instantly. No settings required. No captcha.