Prometheus is an open-source systems monitoring and alerting toolkit originally built at SoundCloud. It focuses on multi-dimensional time series data for metrics storage and provides a robust query language for accessing this data. Prometheus does not rely on distributed storage and uses a single server node model that is autonomous. Prometheus has a healthy eco-system that provides GUI based dashboards, push gateways for temporary jobs, a command-line query tool as well as many other support tools.
Datadog is a turn-key APM solution that provides aggregation and insight into metrics and events across multiple development stacks. It provides a real-time interactive dashboard builder, full-text structured search across all event data, built-in collaboration tools and alert notifications.
SignalFX is a complete solution that aggregates and analyzes metrics and events needed to detect outliers so as to spot trends before they become problems. SignalFX provides pattern based alerting, built-in dashboards, real-time analytics with an emphasis on making monitoring as automatic as possible.