Fluentd

It takes logs in from applications, servers, containers, and cloud services, transforms them if needed, and routes them to whatever storage or analysis tool you use.

Fully free under Apache 2.0. CNCF graduated project. 700+ community plugins cover every source and destination you can think of. The architecture is simple: input plugins (where logs come from), filter plugins (transform/parse), output plugins (where logs go).

Treasure Data (the company behind Fluentd) offers enterprise support and their own managed log analytics platform, but Fluentd itself is completely free.

The catch: Fluentd is written in Ruby, and for high-throughput scenarios, it can be resource-heavy. That's why Fluent Bit exists, a lightweight, C-based alternative from the same project. For Kubernetes, most people run Fluent Bit as a DaemonSet (one per node) that forwards to a central Fluentd instance. The plugin ecosystem is powerful but plugin quality varies; some community plugins are abandoned. And debugging Fluentd configuration issues when logs aren't flowing is tedious.