"a service you can count on"? - free as in freedom unique number repository (web service and client) available
Every needed a service that takes care of ensures each number is unique?
The unique number repository is such a service. Since it is "free as in freedom", you can own your data (why? because it is your data of course).
What is it? A simple REST based server and an example command line client.
It is using an authentication token to prevent the service is used by unauthorized persons. Feel free to create a pull request for a more sophisticated authentication method.