Another real-time search and indexing system built on Apache Lucene.

From their website:

Zoie is a mature open source project and has been deployed in a real-time large-scale consumer website: LinkedIn.com handling millions of searches as well as hundreds of thousands of updates daily.

All Zoie releases have gone through extensive functional and performance testing by LinkedIn before made public. All major versions are released after a trial period on the production environment.

In a real-time search/indexing system, a document is made available as soon as it is added to the index. This functionality is especially important to time-sensitive information such as news, job openings, tweets etc.