In many ways, security starts with understanding the situation. For a couple of generations of IT security professionals, understanding their networks' vulnerabilities starts with Nessus from Tenable. According to sectools.org, Nessus is the most popular vulnerability scanner and third most popular security program currently in use.
Nessus comes in both free and commercial versions. The current version, Nessus 7.1.0, is a commercial program, though it is free for personal home use. Version 2, which was current as of 2005, is still open source and free.
Tenable has maintained Version 2, though in the best tradition of open source software, it has forked and developed in several different directions. Nessus knowledge remains a valuable career skill.