uQuery.com is an awesome and powerful free search engine for mobile apps. Tire and ElasticSearch are used in parts of our infrastructure.
Ataxo Social Insider is is a Rails application for monitoring and evaluating communication in social networks. Whole application is powered by ElasticSearch via Tire: see our mini case study at ElasticSearch's website.
Graylog2 is a free and open source log management solution that uses ElasticSearch (with Tire) as message backend from version 0.9.6 on.
Skype in the classroom is a collaboration platform for students and teachers.
GL Stock Images is a royalty free stock photography marketplace that uses ElasticSearch with Tire to provide high quality search results and image recommendations for customers. After using several different search engines, ElasticSearch and Tire proved to be the best solution for the most relevant and flexible search results.
InEx Finance is a nice free web application for personal money management and budgeting. ElasticSearch and Tire are used to deliver relevant quality results for 4 search types: sitewide search, transactions search within account, page search and questions/answers search.
Tabeso is a mobile application that allows you to find events and things happening around you, and share them with your friends. You can follow venues, and performers to keep up-to-date with their events. Many more features coming as the app progresses.
Preserve.io is a bookmarking tool which downloads copies of your bookmarks to make sure they're always available when you need them and allows you to search through them later.