If you're using micro-services, what are you favorite micro-services that are really on the "micro" end of the scale?

I'm thinking about the self-contained services that have a couple endpoints. The things that do something really well and make developing across your code much easier.

Two that come to mind for me are:

- A progress-bar API that was delightful when you wanted to give users a sense of where they were in a long running process on a distributed system.

- A rate limit service that was amazingly flexible and let me have tens of millions of individual token buckets.