Verifying an address matches its stated postal code before checkout
A mismatched postal code and city on an order form looks like a small typo until it turns into a delivery sent to the wrong part of the country entirely.
Adding a country flag and local time to user profiles
A remote team collaboration tool had a profile field for time zone that almost nobody filled in correctly, since it required a person to manually select their own time zone from a long dropdown list during onboarding, a step easy to skip or get wrong, especially for someone who had just moved or was traveling when they set up their account. The result was a team directory full of stale or missing time zone data that nobody trusted enough to actually use when deciding whether to message a colleague.
The company replaced the manual field with an automatic one. Rather than asking a user to select their time zone, the app resolved it directly from the connection making the request. /v1/ip took the user's IP address at login and returned country and coordinates, and those coordinates fed into /v1/timezone, which returned the IANA time zone name for that location, kept fresh by re-resolving periodically rather than locking in whatever a user's location happened to be the one time they filled out a form.
The visible result in the product was a small addition to every teammate's profile: a country indicator and a live local time, updating automatically, next to their name in the team directory and in any direct message conversation. A colleague deciding whether to send a message or wait until morning could see, at a glance, whether it was a reasonable hour where the other person was, without needing to remember which country they were based in or do any time zone math themselves.
The company kept this adjustable rather than fully automatic and unchangeable, since a user occasionally did want to override the detected location, someone working temporarily from a different country than they normally would, for example, and preferring their profile to reflect their usual time zone rather than wherever they currently happened to be connecting from. The automatic detection set a sensible default that updated on its own for the common case, while a manual override remained available for the exception.
This was a genuinely small feature in terms of engineering effort, essentially two lookups chained together at login and a small display change in the team directory, but it removed a piece of friction that had been quietly costing the team small amounts of coordination overhead for years, the low-grade cost of not knowing whether now was a good time to reach out to someone without asking first or just guessing.
Volume tracked login sessions rather than every single message or page view, since the location and time zone data was refreshed periodically rather than on every request, keeping the workload well within the free daily allowance even for a team collaboration tool with a meaningfully large user base logging in throughout the day.
Small, quiet defaults like this tend to matter more cumulatively than any single flashy feature, since they remove a tiny bit of friction from something that happens constantly rather than solving a problem that only comes up occasionally. Documentation for both endpoints is at /docs/ipv4-lookup/ and /docs/timezone-lookup/.