The Gigablast search-engine has open sourced its search-engine code and technology at GutHub…

“An open source web and enterprise search engine for Linux on Intel/AMD. Currently in use on A robust, scalable search solution in 100% custom C/C++ that has
been in development and used commercially since 2000. Distributed web crawler. Supports any document conversion plugin to convert PDF, etc. to HTML”

Code currently lacking, though, any “Boolean query support”. That probably doesn’t matter, though, since only a miniscule fraction of seachers actually use Boolean.