Snort

Snort is a free and open source network intrusion prevention system (NIPS) and network intrusion detection system (NIDS) created by Martin Roesch in 1998. Snort is now developed by Sourcefire of which Roesch is the founder and CTO. In 2009 Snort entered InfoWorld's Open Source Hall of Fame as one of the "greatest [pieces of] open source software of all time".