VSCode is an excellent & free code editor from Microsoft.
Atom Text Editor
Atom is another great free text editor perfect for coding sites.
Can I use ____?
Browser Support information on modern techniques, such as Flexbox support.
DNS, SPF and other debug tools
A collection of tools for checking DNS & email related issues.
Usage information for new HTML5 tools, with links to Polyfills where needed.
Copy and paste your css here to make sure you've got all cross browser prefixes sorted, also available as plugins for most text editors.
Find out fine grained details of CSS properties that cause layout, paint, and composite in the browser, useful in combination with Chrome Dev Tools to fix Jank on your pages