ThingsBoard is the most mature IoT platform with public source code: device management, rule engine, dashboards with 600+ widgets and native multi-tenant capability - all in one system. Actively developed, production-proven, over 15,000 GitHub stars.
ThingsBoard is, alongside Grafana, one of the core foundations on which we build IoT platforms. We use it not as an isolated tool, but as a central component in the overall stack: custom rule chains, White-Label interfaces, API integrations and managed hosting from a single source.