At Migroot.io, we built a simple free feature that checks whether your country has a double taxation agreement with the one you’re moving to.
It was meant to be just a small helper for our relocation tracker but people loved it. It became one of the main ways new users discover us.
Now we’re thinking of turning it into a proper sub-feature inside the main product, so taxes and visas live in one place.
What other small but useful things have you added to your product that ended up working better than planned?

It is indeed a helpful feature, and the interface is so smooth.
People often underestimate the importance of simplicity in a product. This tax feature is definitely very helpful and straightforward and simple to use. I remember to make a simple tool to track flights(whereflight) and people loved it because of the easy interface. Appreciate your thought on making this. Kudos!