Woodpecker

Woodpecker is a self-hosted CI engine that's dead simple to set up. It's a community fork of Drone CI, after Drone went commercial. Apache 2.0, Go. The YAML pipeline syntax is nearly identical to Drone's. Docker-based execution means each step runs in an isolated container. Supports GitHub, GitLab, Gitea, Forgejo, and Bitbucket. Multi-platform: Linux, ARM, Windows agents.

Fully free. No paid tier, no premium features, no hosted offering. Everything (parallel pipelines, matrix builds, secrets management, cron jobs) is included. Self-hosting is straightforward. One server + one or more agents. A small VPS ($5-10/mo) handles most projects. Docker Compose gets you running in under 15 minutes. The ops burden is light. It's a Go binary, not a Java monstrosity.

Solo to small teams: perfect. You get unlimited CI minutes for the cost of a cheap VPS. Medium teams: works well with multiple agents for parallel builds. Large teams: possible, but you'll miss the plugin ecosystem and enterprise features that Jenkins or GitLab CI offer.

The catch: the plugin ecosystem is smaller than Drone's was, and vastly smaller than GitHub Actions or GitLab CI. If you need a niche integration, you might be writing your own. And the community, while active, is small. Fewer tutorials and Stack Overflow answers.