DNSstuff - 10 Best Free and Open-Source SIEM Tools
This article lists Security Information and Event Management (SIEM) tools that provide log analysis and correlation of events. Caveat: Open source may be sufficient for some small companies but do not provide support and may offer only a limited feature set. Most of these open source solutions offer a paid option as well. If you try it and like it, upgrading to the paid option to gain support and features is easy.