Jenkins is an open source automation server written in Java. It is used to continuously build and test software projects, enabling developers to set up a CI/CD environment. It also supports version control tools like Subversion, Git, Mercurial, and Maven.
Jenkins is well-documented and extremely extensible, with a rich ecosystem of plugins and integrations. For that reason, it excels when used in large projects that require lots of customization.
While Jenkins itself is free, it must be run on a server which will need attention, updates, and maintenance.