LiveMon!

An uncompromising mature enterprise-level platform designed for real-time monitoring of millions of metrics collected from tens of thousands of servers, virtual machines and network devices, effortlessly scaling to even larger environments.

Datadog is a monitoring service for IT, Dev & Ops teams who write and run applications at scale, and want to turn the massive amounts of data produced by their apps, tools and services into a unified view of IT infrastructure. Datadog's capabilities are provided on a SaaS-based data analytics platform that enables multiple teams to work collaboratively on infrastructure issues.

Shinken is an open source computer system and network monitoring software application compatible with Nagios. It's written in Python and therefore is able to run on all platforms where a Python interpreter is available.In contrast to Nagios it is not a monolithic system. Shinken runs as a bunch of highly specialized processes, avoiding the bottlenecks of a single-process design. This parallelity allows to run Shinken not only on one single server. It's components can be distributed over a whole server farm, which results in incredible scalability.Shinken is actively developed and already got more features than Nagios (like priorities for objects, business processes, auto-generation of services, discovery,...) »

Opsview Atom is ideal for personal use, SoHo setups, test or lab environments or just to get your hands dirty with all of Opsview's monitoring functionality!

Monitis is a Cloud monitoring and IT analytics service offering Website Monitoring, Server Monitoring, Network Monitoring, Application Monitoring, Cloud Monitoring, and Custom Monitoring.

Librato is a cloud-based monitoring platform for devops, development and operations teams who want the flexibility to monitor the metrics and events important to their application deployment, while leaving storage, analysis and alerting to a service that can scale with their operation.

Opsview is award winning unified service monitoring software for complex, distributed IT infrastructures.

RapidSpike is an uptime and performance monitoring service for websites and web applications. It simplifies monitoring, alerting and reporting to support your online presence and to ensure a positive user experience. Organisations use it to identify problems, measure performance and improve capacity planning.

FREE online web application that allows you to monitor your server/website: response times, availability, website statistics. Very easy in use even for non-technical people.

Dynatrace Ruxit provides all-in-one monitoring and operations management for DevOps teams working in startups, mid-sized companies, and large organizations. Using proprietary artificial intelligence technology, Ruxit enables companies to manage thousands of servers using a simple interface.