Jenkins

Jenkins is the most widely adopted open-source automation server, used by millions of developers worldwide. It provides hundreds of plugins to support building, deploying, and automating any project.

The platform supports declarative and scripted pipelines defined as code (Jenkinsfile), enabling version-controlled build definitions. It distributes work across multiple machines for faster builds with its agent architecture.

Jenkins integrates with virtually every source control, build tool, and deployment target in the industry. The plugin ecosystem covers everything from Docker to Kubernetes to cloud providers.

While newer tools offer more opinionated workflows, Jenkins remains dominant in enterprises due to its flexibility, extensive plugin library, and zero licensing cost.

Free: Open source under MIT license

CloudBees CI (commercial): Custom pricing for enterprise features

Self-hosted, so infrastructure costs apply.