I'm thinking of building an application that shows you the nearest ATM machine, but I don't know where I can find open data about ATM locations (or if there even is such data available).
I'm asking for Paris because I'm thinking of starting the initial version of the app from here.
OpenStreetMap's database has the ATM tag. Through the Overpass API, you can quickly access the Points of Interest: Map of ATMs in Paris (Overpass API). Click on the points to see additional metadata (mainly the name of the operating bank).
Bonus: there is a good thread on XAPI call for all ATMS in OpenStreetMap's own Q&A plattform.
You can use the Google Places API which has atm as a place type. You might also want to do some research around existing applications which already provide the functionality you describe.
It seems that you could be interested by the MasterCard SDK, specifically the ATM Locations web service. You would first need to register to the MasterCard Developer Zone, which will allow you then to register to one of the services available, among them, the ATM Locations web service.
The SDK is available for the Java language and provided that you properly set the authorization header in the HTTP request, getting ATM locations for a particular region seems to be relatively easy:
https://api.mastercard.com/atms/v1/atm?Format=XML&PageOffset=0&PageLength=10&AddressLine1=70 Main St&PostalCode=63366&Country=USA
There also seems to be a Visa Developer Program, with an ATM Locator Service available, but registration and access to the service is subject to Visa approval, see:
If you are not a Visa client, you will be contacted to determine whether you are eligible to use the API. Your participation and access to the API is subject to Visa approval.
Note that both of these SDK's are not that much Open Data, but they could be useful in case you do not get enough information with the Overpass API, or the Google Places API, respectively mentioned by @odjo and @FreshPrinceOfSO.
