Ask HN: Dear startup founders, what have you developed in-house?

I have been discussing this topic with several founders and have come to realize that when startups develop non-core solutions in-house to address specific problems, these often present very interesting business opportunities. This is especially true if the solutions are complex and time-consuming to develop. Even if third-party software that addresses these problems is expensive, it's usually still more cost-effective than dedicating engineering resources to develop similar solutions in-house.

I want to test this thesis with the Hacker News community of startup founders by asking about solutions they have developed in-house and their reasons for doing so.