Segment is a customer data platform (CDP) that helps you collect, clean, and control your customer data.segment.com
New Relic's application monitoring gives you detailed performance metrics for every aspect of your environment. Manage application performance in real-time. New Relic is the market leader, but tends to be on the more expensive end.
Split provides a unified solution for feature flags, experimentation and continuous delivery, centralizing feature flag management. Split has a more complex offering than just feature flagging that includes A/B testing and experimentation. Has a free tier.