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.
SigNoz - Open source alternative to Datadog
CatchJS - Unbloated, yet feature rich. CatchJS provides JavaScript Error Tracking the way it should be.
Librato - Your monitoring infrastructure in the cloud. Store, visualize and alert on all your metrics at all levels of the stack. Get your 30d free trial.
AirBrake - Airbrake is the leading exception reporting service, currently providing error tracking.