Here is a collection of tools I’ve found useful.

Performance

• Google PageSpeed Insights: pagespeed.web.dev
• GTmetrix: gtmetrix.com
• WebPageTest: webpagetest.org (Useful for waterfall chart and list of all loaded files)

Social

• Preview Open Graph data for any page: omgimg.co/social/preview

Images

• Convert SVG to CSS: yoksel.github.io/url-encoder

Plugin development

• Check to make sure your plugin actually works with the minimum WP version it declares: https://github.com/eduardovillao/wp-since