Snort

OpenVAS is a full-featured vulnerability scanner that includes unauthenticated and authenticated testing, various high-level and low-level internet and industrial protocols, performance tuning for large-scale scans, and a powerful internal programming language to implement any vulnerability test.

Sniffnet is an open-source, cross-platform application to monitor your network traffic. It allows you to set filters to apply to the observed traffic, views overall statistics, set custom notifications, and more.