Nessus is the best free network vulnerability scanner available, and the best to run on UNIX at any price. It is constantly updated, with more than 11,000 plugins for the free (but registration and EULA-acceptance required) feed. Key features include remote and local (authenticated) security checks, a client/server architecture with a GTK graphical interface, and an embedded scripting language for writing your own plugins or understanding the existing ones. Nessus 3 is now closed source, but is still free-of-cost unless you want the very newest plugins.