Zookeeper is one of the best open source server and service that helps to reliably coordinates distributed processes. Zookeeper is a CP system (Refer CAP Theorem) that provides Consistency and Partition tolerance. Replication of Zookeeper state across all the nodes makes it an eventually consistent distributed service.
Moreover, any newly elected leader will update its followers with missing proposals or with a snapshot of the state, if the followers have many proposals missing.
Zookeeper also provides an API that is very easy to use. This blog post, Zookeeper Java API examples, has some examples if you are looking for examples.
So where do we use this? If your distributed service needs a centralized, reliable and consistent configuration management, locks, queues etc, you will find Zookeeper a reliable choice.