UI Cloud - The largest user interface design database in the world
Apache Lucene - High-performance, full-featured text search engine library written entirely in Java.
Reactive Search - UI components for building Amazon / Yelp like Search
Six Degrees of Wikipedia - Find the shortest path between two subjects
OpsDash - All-in-one solution for monitoring servers, services, uptime and databases. Curated Dashboards. Rule-based alerting.
Sphinx Search - Sphinx is an open source full text search server, designed with performance, relevance (search quality), and integration simplicity in mind. Sphinx lets you either batch index and search data stored in files, an SQL database, NoSQL storage.