OpenVAS (Open Vulnerability Assessment System) is a set of tools offering vulnerability scanning and vulnerability management. The development of OpenVAS, which is a fork of Nessus Vulnerability Scanner, started in 2005 after the developers of Nessus changed the license to a proprietary closed source license.
OpenVAS allows you to run several network vulnerability tests (NVT) written in Nessus Attack Scripting Language (NASL), which OpenVAS gets through a public NVT feed. The feed contains more than 50.000 NVTs.
All OpenVAS components are free software and can be downloaded from http://www.openvas.org