Open source search engines allows anyone who desires to make changes for the improvement of the software. It is an approach to contribute improvements and additional functionality to the software.
There are several open source search engines available to download and use. Here is the list
ASPSeek
BBDBot
Datapark
ebhath
Egothor
Eureka
Gonzui
Grub
ht://Dig
Indri
ISearch
IXE
Lucene
Managing Gigabytes (MG)
MG4J
mnoGoSearch
MPS Information Server
Namazu
Nutch
Omega
OmniFind IBM Yahoo! Ed.
OpenFTS
PLWeb
Sciencenet
Sphinx
Sqlite
SWISH-E
SWISH++
Terrier
WAIS/ freeWAIS
WebGlimpse
Xapian
XML Query Engine
XMLSearch
YaCy
Zebra
Zettair