Fluentd is an open source log aggregator.
Fluentd is a fully free and fully open-source log collector that instantly enables you to have a "Log Everything" architecture with 125+ types of systems.
Fluentd treats logs as JSON, a popular machine-readable format. It is written primarily in C with a thin-Ruby wrapper that gives users flexibility.
Fluentd's performance has been proven in the field: its largest user currently collects logs from 5000+ servers, 5 TB of daily data, handling 50,000 msgs/sec at peak time.