elmah.io - The coolest cloud based error management system for .NET web applications. Based on the de-facto standard error component ELMAH, logging errors from your webserver have never been this easy. Full-text search, notifications, API and much more.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Raygun - Raygun gives developers meaningful insights into problems affecting their applications. Discover issues - Understand the problem - Fix things faster.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Sentry.io - From error tracking to performance monitoring, developers can see what actually matters, solve quicker, and learn continuously about their applications - from the frontend to the backend.
Prometheus - An open-source systems monitoring and alerting toolkit.