Kiwix is an offline reader for Web content. It was designed for use with Wikipedia, but is potentially suitable for all HTML content. It supports the ZIM format, a highly compressed open format with additional meta-data. It is intended for use in schools, universities, and libraries which can't afford broadband Internet access. It features a full text search engine, bookmarks and notes, an HTTP server, PDF/HTML export, a user interface in more than 80 languages, tabbed navigation, and an integrated content manager and downloader.
XapianFu is a Ruby library for working with Xapian databases. It builds on the GPL licensed Xapian Ruby bindings, but provides an interface more in-line with "The Ruby Way"(tm) and is considerably easier to use. For example, you can work almost entirely with Hash objects, and XapianFu will handle converting the Hash keys into Xapian term prefixes when indexing and when parsing queries. It also handles storing and retrieving hash entries as Xapian::Document values. XapianFu basically gives you a persistent Hash with full text indexing (and ACID transactions).
RestPose is a search engine. It is designed to take a set of documents and then, when given a query, to return ranked lists of documents which are a good match for that query. RestPose manages a set of internal indexes and provides an interface (over HTTP, in a fairly RESTful style, using JSON as the main transfer format) which allows documents to be submitted and removed from indexes, and which allows searches to be performed.