This document provides an overview of open source library-related software. It discusses the growth of open source in libraries due to more IT expertise among library professionals. It then describes several types of open source library software, including integrated library systems (ILS) like Koha and Evergreen, online public access catalogs (OPACs) like VuFind, digital collection management systems like Greenstone and Fedora, and metasearch tools like Libraryfind. It also discusses open URL resolvers and other smaller, locally developed projects. The document notes some considerations for open source software like requiring technical expertise, data conversion, training and documentation. It discusses support and maintenance models for open source and mentions commercial support services provided by companies like Liblime.