Some developer tools that spun out of my current product (but not launched thus counts as "built for myself" and nowadays "ourselves"):
- A build system that is simpler and easier to use than Gradle. It also has a much better rendering of progress and output for tests when they fail. I mostly use it to run tests, actually. Supports parallel incremental and cached work.
- A Kotlin Scripting runtime that exposes a high-level UNIX shell-like API along with many other useful utilities like printing markdown, tracking progress of long operations (including hierarchically), working with files and remote programs over ssh and many other things. We have almost entirely replaced bash with it.