One of the best tools on linux is logwatch.  This little tool holds so much power and has more than once shown me that something or someone was trying to hack a system on the sitting on the Internet Directly.  While no tool is fool proof this one is reliable and can be tuned to show you just what you care about.  They have a great series going right now over at Linux Mag that explains some of the basics of this wonderful tool.  They also have other links to similar articles about taming your logs.