Snort is an open-source intrusion detection system (IDS) created by Sourcefire. It is also one of the most widely deployed open source security tool, with more than 3,000,000 downloads to date. Installing Snort is quick and painless. However, here's a tutorial on how to further improve with Barnyard, a tool to speed-up Snort output data.
For more tutorials, how-tos, whitepapers and other documents, check out the Snort Documentation page.
[image courtesy of Brent & MariLynn]