TestLink is an open source test management tool written in PHP. In order to install TestLink you will need a HTTP server with PHP 5 and a database. The databases currently supported by TestLink out of the box are MySQL, Postgre SQL and MS SQL, although there are users that managed to use Oracle too. You can download it from http://www.teamst.org.
In TestLink you have, among other features, Test Plans, Requirement management, Baselines, Custom Fields, Test Suite with Test Cases and Reporting. For external access, there is an XML-RPC API available (it is not enabled by default). Other two nice features in TestLink are the versioning of the some entities, like Test Case and Requirements, and the ability to import and export data in different formats.
There are bindings for the TestLink XML-RPC API available for Java, Perl, PHP, Ruby and other programming languages. The integration between Jenkins and TestLink is done using TestLink Java API - http://testlinkjavaapi.sourceforge.net.
At the moment that this guide is being written the latest version of TestLink is 1.9.3. While the Jenkins version is not a big issue for this integration, the same cannot be said about TestLink. This guide is intended for Testlink 1.9.3 XML-RPC API. In case there are other versions available at the moment that you are reading this guide, please consult the plug-in Wiki for further information on this.
TestLink is licensed under the GPL License and its code is hosted at gitorious - http://www.gitorious.org/testlink-ga/testlink-code.