Jenkins

Jenkins automates builds, tests, and deployments with 1,800+ plugins covering virtually every tool in the software development ecosystem. Jenkins is the original and still the most extensible. It's been around since 2011 and there isn't a workflow it can't handle. MIT licensed. Jenkins runs builds, tests, and deployments using declarative or scripted pipelines defined in Jenkinsfiles. It scales from a single machine to hundreds of build agents.

Fully free. No paid tier. CloudBees offers a commercial Jenkins distribution, but the community edition is complete.

The catch: Jenkins is showing its age. The UI looks like 2012. Configuration is often click-through rather than code-first. Maintaining a Jenkins server means updating plugins constantly (some of which conflict), managing security patches, and dealing with Java memory tuning. Modern alternatives like GitHub Actions, GitLab CI, or ArgoCD handle common workflows with less operational burden. Jenkins still wins for complex, custom pipelines that nothing else can express, but if your needs are standard, you're paying an ops tax for flexibility you don't use.