Screwdriver CD is an open-source continuous delivery platform open-sourced by Yahoo in 2015, offering container-native builds, declarative YAML pipelines, and auto-scaling.
Drone CI is a lightweight container-native CI/CD platform under Harness, known for single-binary deployment and simple YAML configuration, running each build step in its own Docker container with support for GitHub, GitLab, and Bitbucket.
Concourse CI is a container-native open-source CI/CD system initiated by Pivotal (now Broadcom/VMware), using a resource-oriented pipeline model where each task runs in an isolated container, known for declarative YAML and reproducible builds.