ConvertKit
ConvertKit is the only email marketing platform I use. I tested several ones and ConvertKit is so much better, not only as a mailing platform, but also when it comes to fueling your Customer Success. I can't imagine switching to something else.
Integromat
Who would have thought that a software engineer would like to use a no-code tool? If you ever used some kind of integration tooling, Integromat brings it to another level. If you haven't, you need to try it anyway.
Nozbe
I've been using this tool for a few years now. You can say, it's just another To Do List, but it works where and when I need it.
Instapaper
Probably TOP 1 tool, beside all the books, that supports me in my learning. In 99% of cases I don't read an article immediately, but add it to Instapaper instead. This separates planning/grooming from the actual reading action.
GitHub
I use it for so many purposes. Keeping my code, keeping a repo of blog posts as well as the source code for this site. GitHub Pages host this blog. Majority of my projects uses now GitHub Actions to provide CI and CD for them.
Jekyll
Jekyll is the static page generator that use frequently, for instance to create this blog. With the features like collections, data, nested layouts, one can do a lot. If you add just a handful of serverless components you can have a full blown page ready without any server at all.
Cloudflare
Not only the best DNS on the market but also top notch caching and page rules that allow so many things. Recently I started using Cloudflare Workers for some things, so if you read this blog, you're probably using them from time to time.