ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.
Bloomerang - Bloomerang is a simple donor database and fundraising software solution that helps nonprofits decrease donor attrition and increase revenue.
Xapian - Xapian is an open source probabilistic information retrieval library, released under the GNU...
Kindful - Nonprofit donor database + fundraising tools all in one
Algolia - Algolia's Search API makes it easy to deliver a great search experience in your apps & websites. Algolia Search provides hosted full-text, numerical, faceted and geolocalized search.
Classy - Expressive, flexible, and powerful stylesheets for native iOS apps