Search Engines
DataparkSearch Engine is a full-featured open sources web-based search engine released under the GNU General Public License and designed to organize search within a website, group of websites, intranet or local system.
More Info 
Developer: Maxim Zakharov
License: GNU General Public License (GPL)
Operating System: Platform Independent
ASPSeek is an Internet search engine.
Developer: SWsoft
License: GNU General Public License (GPL)
Operating System: Platform Independent
Doodle is a tool to quickly search the documents on a computer.
Developer: Christian Grothoff
License: GNU General Public License (GPL)
Operating System: Not Available
Xapian is an Open Source Search Engine Library, released under the GPL.
Developer: Olly Betts
License: GNU General Public License (GPL)
Operating System: All 32-bit MS Windows (95/98/NT/2000/XP), All POSIX (Linux/BSD/UNIX-like OSes), OS Portable (Source code to work with many OS platforms), OS X