The CDL eXtensible Text Framework (XTF) is a flexible indexing and query tool that supports searching across collections of heterogeneous data and presents results in a highly configurable manner.
The Mycroft project provides a collection of Sherlock & OpenSearch Search Engine Plugins (15272 at the last count) for your web browser. It is part of the Mozilla project.
Xapian is an Open Source Search Engine Library, released under the GPL. It's written in C++, with bindings to allow use from Perl, Python, PHP, Java, Tcl, C# and Ruby (so far!)
A standalone search engine, meant to provide fast, size-efficient and relevant fulltext search functions to other applications. Sphinx was specially designed to integrate well with SQL databases and scripting languages.