I recently came across some conversations on the Perchology Slack channel discussing the Cleantalk Antispam plugin and decided to give it a try on my website EICAC which had been sending me quite a bit of spam through the contact form.
I followed the installation procedure which was very simple. Download the universal plugin, upload to a Cleantalk directory on your domain and follow the installation instructions. I did have an issue with the plugin installation, which was caused by me rather than the plugin but the support is fantastic and they completed the installation for me.
With the plugin having been running for over two weeks now, I have seen no spam get through and logging on to my admin panel can see the log of stopped spam is quite extensive.
Cleantalk is fantastic value, especially if you are a developer and add it to multiple websites as an addon for clients. I would highly recommend it for an Antispam alternative to Recaptcha.
Update: As of March 2021 I am still seeing fantastic results from incorporating Cleantalk and have added it to two other client websites which were seeing a reasonable amount of spam, both of which have seen a big reduction.