Wandercoin

Wandercoin is the app you reach for when:
You're standing in front of a price tag in another country and want to know whether 1,800 yen is closer to 11 or 18 euros.
You're about to buy a camera lens from a Japanese website and want to see what your card will actually be billed.
You're booking an apartment in Greece for September and wondering whether the dollar is going to wreck the price by then.

You're back from a trip and the credit card statement landed with rates that look optimistic. You want to check what the rate actually was on the day you paid.

How it does that
Pick two currencies and type an amount on either side. The other side updates as you type. Star the currencies you use most often so they sit at the top of the picker.

Under the conversion is a chart of how the pair has moved over the last week, month, three months, six, a year, or five. Drag your finger across it to see the exact rate on any past day, with a small haptic tick. Useful for sanity-checking that credit card statement, or for getting a feel of whether to exchange now or wait.

Where the rates come from
The European Central Bank publishes one reference rate per workday, around 4 PM Berlin time. Wandercoin pulls them from the open Frankfurter API and caches them on your device. No weekend updates, because the ECB doesn't publish on weekends. No live tick-by-tick rates either. If you trade forex by the second, this is the wrong tool.

What it doesn't do
No account, no signup, no email. No ads, no analytics, no third-party tracking. The currencies you pick, the ones you star and the last amount you typed all stay on your device and disappear when you uninstall.

Crypto and precious metals are not included, because the data source doesn't cover them.

The data source is open and public, and the source code is on GitHub if you want to look under the hood.