strus is described as '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' and is an app in the Development category. There are four alternatives to strus for a variety of platforms, including Linux, Mac, Windows, BSD and Microsoft Azure. The best alternative is Apache Solr, which is both free and Open Source. Other great apps like strus are Sphinx (search engine) (Free, Open Source), Xapian (Free, Open Source) and Cludo Site Search (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.