Celery - Asynchronous task queue/job queue based on distributed message passing. It is focused on real-time operation, but supports scheduling as well.
Ajenti - Manage a remote Linux box at any time using everyday tools like a web terminal, text editor, file manager and others.
ProxMox Virtualiation Platform - Open-source platform for enterprise virtualization that tightly integrates KVM hypervisor and LXC containers, software-defined storage and networking functionality on a single platform, and easily manages high availability clusters and disaster recovery tools with the built-in web management interface.
NsLookup.io - Simple service that shows all DNS records for a domain name
netshoot - a Docker + Kubernetes network trouble-shooting swiss-army container
netdata - Distributed real-time performance and health monitoring.
Grafana - Open source software for time series analytics.
monit -Small Open Source utility for managing and monitoring Unix systems. Monit conducts automatic maintnance and repair and can execute meaningful causal actions in error situations.
Prometheus - Open-source systems monitoring and alerting toolkit originally built at SoundCloud.
sensu - Monitor servers, services, application health, and business KPIs. Collect and analyze custom metrics. Get notified about failures before your users do. Give your business the competitive advantage it deserves. (Open Source or Commercial)