Nessus was a popular free and open source vulnerability scanner until
they closed the source
code in 2005 and removed the free
"registered feed" version in 2008. A limited "Home Feed” is still
available, though it is only licensed for home network use. Some people
avoid paying by violating the "Home Feed” license, or by avoiding feeds
entirely and using just the plugins included with each release. But
for most users, the cost has increased from free to $1200/year. Despite
this, Nessus is still the best UNIX vulnerability scanner available and
among the best to run on Windows. Nessus is constantly updated, with
more than 20,000 plugins. 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