Xapian is described as 'open source probabilistic information retrieval library, released under the GNU General Public License (GPL). It full text search engine library for programmers' and is an app in the Development category. There are more than 10 alternatives to Xapian for a variety of platforms, including Linux, Windows, Mac, Self-Hosted solutions and Online / Web-based. The best alternative is elasticsearch, which is both free and Open Source. Other great apps like Xapian are Algolia (Free Personal), Apache Solr (Free, Open Source), Sphinx (search engine) (Free, Open Source) and wordpress i-search pro (Paid).
Solr is an open source enterprise search server based on Lucene search library, with XML/HTTP and JSON APIs. Its major features include powerful full-text search, hit highlighting, faceted search, dynamic clustering, database integration, and rich document handling.
Sphinx is a free software search engine designed with indexing database content in mind. It currently supports MySQL, PostgreSQL, and ODBC-compliant databases as data sources natively. Other data sources can be indexed via pipe in a custom XML format.
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.
The open source (GPL v3) project strus provides a collection of programs and libraries (language C++) for building a competitive scalable full-text search engine. The search index is distributable and user rights can be implemented.