Koha, available at http://koha-community.org, is an open source Integrated Library System.
The Koha software has "forked" so be sure you know what you're getting before you start downloading and installing anything.
Koha is a very robust free and open source software system. Koha requires a fair amount of IT skill to install so you should read the documentation carefully before attempting to install the software.
A list of software support vendors who can help you install and maintain a Koha system is available at http://wiki.koha-community.org/wiki/Category:Support_Companies.
GHW - updated 2014.03.05