Code related
https://testing-playground.com/ - This website helps you to visually understand how to unit test using @testing-library
https://checklyhq.com/ - Monitoring as Code workflow for developers: programmable, fast, reliable.
https://readme.com/ - It transforms static API documentation into real-time, interactive developer hubs
https://web.dev/ - Cases of study by Google DevRel
https://ihateregex.io/ - Get a list of regex ready to use, with visual example
Marketing
https://mailchimp.com/ - Handle big list of emails with useful marketing tools
http://keywordtool.io/ - Helps you to better marketing your SEO based on keyword around world, specific country and language
https://www.semrush.com/ - This website helps you to track every public analytics related to specific website, powerful tool to use together with keyworldtool.io
https://marketingplatform.google.com/ - Google Analytics
Tools
https://www.domcomp.com/ - Finds you all possible domains with a name, with the prices and where to buy it
https://www.hotjar.com/ - It gives you an at-a-glance overview of how people interact with a page