I've written about NSMnow a few times before and I'm a big fan. They already had at least partial support for Fedora and I suggested to the developers some changes that would allow them to also support RHEL/CentOS. The SecurixLive team has done an amazing job with NSMnow (and Barnyard2) and things just keep getting better!
You can read more at the SecurixLive site:
Install Sguil on Fedora/RHEL/CentOS using NSMnow