SearchUnit!

ElasticSearch is a distributed, RESTful, free/open source search server based on Java and under the Apache License ( Lucene ).

Xapian is an open source probabilistic information retrieval library, released under the GNU General Public License (GPL). It is a full text search engine library for programmers. It is written in C++, with bindings to allow use from Perl, Python, PHP, Java, Tcl, C#, Ruby, and Lua. Xapian is highly portable and runs on Linux, Mac OS X, FreeBSD, NetBSD, OpenBSD, Solaris, HP-UX, Tru64, IRIX, Microsoft Windows, GNU Hurd, and OS/2.

OpenSearchServer is an open source application server allowing development of index-based applications like search engines. Main features of OpenSearchServer are : An integrated crawler for databases, web pages and rich documents; a user friendly GUI allowing development of most applications with some clicks through an interface built in Zkoss; snippets; faceting; an html renderer for integrating search results in a page; and monitoring and administration features.

SearchBlox is an out-of-the-box Enterprise Search Solution built on top of Apache Lucene/Solr. It is fast to deploy, easy to manage and available for both on-premise and cloud deployment.

Azure Search makes it easy to add powerful and sophisticated search capabilities to your website or application. Quickly and easily tune search results and construct rich, fine-tuned ranking models to tie search results to business goals. Reliable throughput and storage provide fast search indexing and querying to support time-sensitive search scenarios.

The Google Mini is a smaller and lower-cost solution for small and medium-sized businesses to set up a search engine that allows them to index and search up to 300,000 documents.