posthog

PostHog is an all-in-one developer platform for building successful products. We offer product analytics, web analytics, session replay, error tracking, feature flags, experimentation, surveys, data warehouse, a CDP, and an AI product assistant to help debug your code, ship features faster, and keep all your usage and customer data in one stack. Maintained by PostHog on GitHub, where it has earned 35,484 stars from the community.

It's actively developed around ab-testing, ai-analytics, analytics, and is a solid reference for anyone building with these tools.