Nessus is a free, open-sourced and up-to-date remote security scanner for Linux, BSD, Solaris and some other systems. It is multithreaded, plugin-based, has a nice GTK interface and currently performs over 270 remote security checks.
7a59ffad0892001d699924038bdcd641cc6295a9a77c9abcaefc98e1b9b5dcae