Apache Lucene - High-performance, full-featured text search engine library written entirely in Java.
Findo - Your smart search 🔍 assistant across personal cloud ☁️
Six Degrees of Wikipedia - Find the shortest path between two subjects
Meta Search - Search your Desktop, Google Drive, Dropbox, Gmail, Evernote.
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.
HQ.app - HQ is a central place to find everything.