Site: https://regex101.com/
This is a website sandbox for learning [or testing] regex expressions. You are able to search the command repository for commands that may be of help for you, and then you are able to see your regex working in real-time, when run on provided test strings. This was invaluable when I was learning how to use RegEx, and I still go to this website when I am working on complex RegEx to validate how it will work.
Site: https://ninite.com/
When working on new machines, where it is common to install a bunch of applications all at once; this website can help by bundling applications into one installer. The website only bundles a handful of common, but safe, applications, so it is meant to provide a head start in setting up a new computer.
Site: https://creativecommons.org/
A website used for searching photos with varying types of license restrictions.
Site: https://www.pexels.com/
Website with free stock photos
Site: https://color.adobe.com/create/color-wheel
This is a free tool for coming up with color palettes that can be used in Web and Desktop applications.
Site: https://jsfiddle.net/
This is a small sandbox for learning or testing different web technologies. You are able to have 3 panes for HTML, Javascript, and CSS; a fourth pane will then display the output, based on what you type in the other three panes. This website provides the ability to use numerous frameworks such as JQuery, Angular, React, etc.
Site: https://www.litmus.com/
This is a service that allows for mass testing of emails across multiple devices/operating systems at once. Great for testing responsive emails and if you receive user feedback from a user with a particular type of device. Unfortunately, this is not a cheap service, and is likely best left to businesses.