Applications and organizations using Katta include:

Streamy.com uses Katta, Lucene, Zookeeper, and HBase to power its new realtime search engine.  We currently aggregate hundreds of thousands of RSS feeds as well as tens of thousands of activity streams from services like Twitter and Facebook.  We have several hundred million documents in Katta today and adding nearly a million more each day.

Sharethis.com

Bixo - A distributed web crawler

Deepdyve.com - Ted Dunning: “We are using a variant of Katta as the basis of some of our search at Deepdyve. Very, very nice. …
Much of the simplicity and reliability of Katta is directly inherited from the simple programming model and solid implementation of Zookeeper.”

Please post your use case to the mailing list.