Hub Spot Academy:  

Hub Spot offers many free courses and many of them come with certificates! It is a widely recognized platform known for its marketing automation tools, CRM, and inbound marketing solutions, so earning certifications through HubSpot Academy can demonstrate a candidate's proficiency in these areas. 

Google Digital Garage:

Google offers a free course called "Fundamentals of Digital Marketing," which covers essential digital marketing topics like search engine optimization (SEO), online advertising, and social media. It's a comprehensive course with certification upon completion.

Coursera:
While Coursera offers paid certifications, many universities and institutions provide free access to their digital marketing courses. You can audit the course for free, but if you want a certificate, there is a fee. Some courses are offered by institutions like the University of California, Irvine, and the University of Illinois.

LinkedIn Learning:

LinkedIn Learning offers a selection of free digital marketing courses. While most of their courses are paid, you can take advantage of their one-month free trial to access a wide range of topics, including SEO, content marketing, and social media marketing.

Alison:

Alison offers a range of free courses in digital marketing, including topics such as social media marketing, SEO, and digital advertising. While the courses are free, you may need to pay if you want to receive a certificate.