This is a work in progress:
Contrast Ratio Test This is useful for determining whether the text and background colour works well together from a legibility point of view.
Laravel Cheat Sheet This is outdated but is still one of the best cheat sheet resources available.
Google Developers Console Used to create and track API usage for services such as Google Maps, Calendar, Sheets etc.
Favicon Generator A quick way to generate icons including the typical browser favicon but also various app/device based icons. Simply upload a 512px square image and it'll do the hardwork for you.
Spam checker This tool makes for a good content test - should be used to measure the spam rating of an email. Use the comments returned by the app to make informed decisions about the exact copy used for any emailer.
HTML Entity Codes It is always preferred to use entity HTML codes to ensure that symbols are displayed as intended.
Web Page Test Page speed is a massive consideration for all website builds. Use this tool to measure how website handle TTFB (time to first byte) and many other performance metrics. It should be possible to achieve A grades unless there's a technical or server limitation.
Page Speed Insights Google provide this tool to inform developers of how to improve speed for both mobile and desktop experiences. We typically aim for around 80/100 - achieving 100% is not necessary.
Pingdom Similar to the above two tools but tends to be part of all our speed and performance testing as it is provides somewhat different tests to the others.
Tiny PNG We use TinyPNG for all our website and email builds prior to launch. We see massive improvements to image size, especially images with a transparent background.
Tiny JPG We use TinyJPG for all our website and email builds prior to launch. Typically there is no further image quality loss from running JPG images through this tool.
S3 Console Useful for managing storage when using Amazon S3 for a particular project.
Setting up SSH keys Quite often we need SSH keys, this might be required for a Digital Ocean Droplet or perhaps for committing code using GitLab.
DNS Records Helpful for looking up DNS records when waiting for DNS propagation to complete.
DNS Records Always good to compare DNS records from at least one other resource! This tool tends to output all DNS records.
MX Toolbox Similar to the above but you can search for specific DNS records.
IP Address lookup Used for looking up the hostname/owner of an IP address. Helpful for determining whether an IP address is from a suitable source.
Google Search Console A helpful tool for reviewing a website's health - data reported by Google.
Bing Webmaster Tools This is Bing's equivalent tool for monitoring website health.