AlienVault maintains a freely available SIEM solution called AlienVault OSSIM, downloadable from https://www.alienvault.com/products/ossim.
This is taken from the AlienVault website:
"OSSIM, AlienVault's Open Source Security Information and Event Management (SIEM) product, provides you with a feature-rich open source SIEM complete with event collection, normalization, and correlation. Launched by security engineers because of the lack of available open source products, OSSIM was created specifically to address the reality many security professionals face: A SIEM, whether it is open source or commercial, is virtually useless without the basic security controls necessary for security visibility."
