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.
Fluentd - Fluentd is a cross platform open source data collection solution originally developed at Treasure Data.
Raygun - Raygun gives developers meaningful insights into problems affecting their applications. Discover issues - Understand the problem - Fix things faster.
Splunk - Splunk's operational intelligence platform helps unearth intelligent insights from machine data.
Snare - Snare is well known historically as a leader in the event log space.
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.