Flux CD
GitOps operator for Kubernetes — polls a Git repo, applies the manifests in the cluster, and continuously reconciles drift.
Built by Weaveworks (who coined "GitOps"). Since v2 (2021) modular: source-controller, kustomize-controller, helm-controller, image-automation-controller which can bump image tags back into Git when new builds are published.
Competitor: Argo CD (more UI-heavy, more popular in enterprises). Flux is lighter, more "GitOps purist", and a better fit for multi-tenant (each team owns its GitRepository CR).