SzMake

In English:
No spam in comments. blocked by Invisible internal token-code with ajax.
This plugin blocks 100% of spam messages in the author of the environment.
It blocks spam without using the CAPTCHA input-field.
There is no modification of display the comment form.
(but the commnet which is posted by spammers manually via browser is not blocked by this plugin)
In Japanese:
このプラグインはコメント投稿時にAjaxを使ってスパムロボットによる投稿コメントか判別し自動でブロックするプラグインです。
作者の環境では、今のところこのプラグインで100％スパムBOT投稿がブロックできています。
一般的なスパム対策としてAkismetプラグインがありますがブロックされるのは９割程度で100%は止まりませんでした。
別の方法としてCAPTCHA系のプラグインを使いBot対策する手段もありますが、こちらはほぼ100%スパムBotからの投稿はブロックされますがユーザーに煩わしい確認文字入力に毎回協力してもらう必要がありました。 このプラグインでは、見えない入力欄を用意してコメント投稿時にjavascriptでCAPTCHA入力に変わる固有の確認トークン入力処理をで行うことでスパムBotからの投稿をブロックします。
利用ユーザーのコメントフォームの見え方は変わりません。
(残念ながらこのプラグインではブラウザを介した手入力によるスパム投稿はブロックできません)
<a href="http://wp.szmake.net/sz-comment-filtter/" title="Documentation in Japanese" rel="nofollow ugc">日本語の詳細説明ページはこちら</a>
<h4>Translators</h4>
<ul>
<li>Japanese (ja)</li>
</ul>
<h3>Contact</h3>
email to contact[at]szmake.net 
twitter @sxmtz

Sz Comment Filter

No spam in comments. blocked by Invisible internal token-code with ajax.This is not used CAPTCHA.

webvitaly

kudlav

This is fork of successful Anti-spam plugin v5.5 written by webvitalii, for more info visit <a href="https://github.com/kudlav/anti-spam/" rel="nofollow ugc">GitHub Fork</a>. 
From version 5.6 maintained by kudlav.
<ul>
<li><a href="https://github.com/kudlav/anti-spam/" rel="nofollow ugc">GitHub</a></li>
</ul>
Anti-spam Reloaded plugin blocks 100% of automatic spam messages in comments section and also blocks all trackbacks.No captcha required.
Plugin is simple and easy to use: just install it and it just works.
Blocked comments can be stored in the Spam area and converted to regular comments if needed.
Anti-spam Reloaded plugin is GDPR compliant and does not store any other user data except of the behaviour mentioned above.
Plugin blocks spam only in comments section.. 
Plugin does not block manual spam (submitted by spammers manually via browser).
<h3>Compatibility</h3>
All modern browsers and IE11+ are supported. 
Anti-spam Reloaded plugin works with disabled JavaScript. Users with disabled JavaScript should manually fill current year before submitting the comment.
Server compatibility:
<ul>
<li>WordPress 3.3 – 6.5</li>
<li>PHP 5.6 – 8.2</li>
<li>Doesn’t use jQuery</li>
</ul>
Plugin is incompatible with:
<ul>
<li>Disqus</li>
<li>Jetpack Comments</li>
<li>AJAX Comment Form</li>
<li>bbPress</li>
</ul>
If site has caching plugin enabled and cache is not cleared or if theme does not use ‘comment_form’ action 
and there is no plugin inputs in comments form – plugin tries to add hidden fields automatically using JavaScript.
<h3>How does it work?</h3>
The blocking algorithm is based on 2 methods: ‘invisible js-captcha’ and ‘invisible input trap’ (aka honeypot technique).
<h4>‘invisible js-captcha’</h4>
The ‘invisible js-captcha’ method is based on fact that bots does not have JavaScript on their user-agents. 
Extra hidden field is added to comments form. 
It is the question about the current year. 
If the user visits site, than this field is answered automatically with JavaScript, is hidden by JavaScript and CSS and invisible for the user. 
If the spammer will fill year-field incorrectly – the comment will be blocked because it is spam.
<h4>‘invisible input trap’</h4>
The ‘invisible input trap’ method is based on fact that almost all the bots will fill inputs with name ’email’ or ‘url’. 
Extra hidden field is added to comments form. 
This field is hidden for the user and user will not fill it. 
But this field is visible for the spammer. 
If the spammer will fill this trap-field with anything – the comment will be blocked because it is spam.

Sz Comment Filter vs Anti-spam Reloaded