The best open source alternative to Raygun is GlitchTip, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Raygun and five of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Raygun are Logit.io (Paid, Open Source), log4net (Free, Open Source), NLogEx (Free, Open Source) and Errbit (Free, Open Source).
Logit.io is used for a wide variety of use cases including log management, business intelligence, SIEM, APM, container monitoring, infrastructure monitoring, monitoring website uptime, measuring sales performance @ vulnerability scanning.
Errbit is a tool for collecting and managing errors from other applications. It is Airbrake (formerly known as Hoptoad) API compliant, so if you are already using Airbrake, you can just point the airbrake gem to your Errbit server.