Applications and organizations using ZooKeeper include (alphabetically):
company w/ link if available
use
http://www.deepdyve.com - We do search for research and provide access to high quality content using advanced search technologies Zookeeper is used to manage server state, control index deployment and a myriad other tasks.
katta - Katta serves distributed Lucene indexes in a grid environment. Zookeeper is used for node, master and index management in the grid.
101tec - we do consulting in the area of enterprise distributed systems. We use zookeeper to manage a system build out of hadoop, katta, oracle batch jobs and a web component.
HBase - HBase is the Hadoop database. Its an open-source, distributed, column-oriented store modeled after the Google paper, Bigtable: A Distributed Storage System for Structured Data by Chang et al. We use ZooKeeper for master election, server lease management, bootstrapping, and coordination between servers.
Rackspace - The Email & Apps team uses ZooKeeper to coordinate sharding and responsibility changes in a distributed e-mail client that pulls and indexes data for search. ZooKeeper also provides distributed locking for connections to prevent a cluster from overwhelming servers.
Yahoo! - ZooKeeper is used for a myriad of services inside Yahoo! for doing leader election, configuration management, sharding, locking, group membership etc.
CXF DOSGi - The Distributed OSGi implementation at Apache CXF uses ZooKeeper for its Discovery functionality. See here for more information:
http://cxf.apache.org/dosgi-discovery-demo-page.html