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.
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.
Rollbar - Rollbar collects errors that happen in your application, notifies you, and analyzes them so you can debug and fix them. Ruby, Python, PHP, Node.js, JavaScript, and Flash libraries available.
Graylog - Graylog is an open source log management platform for collecting, indexing, and analyzing both structured and unstructured data.
RootCause - Record, Replay and Reproduce JavaScript Errors
Prometheus - An open-source systems monitoring and alerting toolkit.