Algolia's Search API makes it easy to deliver a great search experience in your apps & websites.
Swiftype is a hosted search solution that lets site owners deliver powerful, customizable search for their website or mobile app with just a few lines of code.
Sphinx is an open source full text search server, designed from the ground up with performance, relevance, and integration simplicity in mind.
BaseX is a very light-weight, high-performance and scalable XML Database engine and XPath/XQuery Processor with Full Text and Update extensions.
Open source search engine library written in C++, with bindings to allow use from other languages as well.
The Lemur Project develops search engines, browser toolbars, text analysis tools, and data resources.
Searchify provides hosted search-as-a-service, with a simple API, flexible results ranking, geolocation, and 100% IndexTank compatible.