OpenVAS evolved from Nessus when Tenable put Nessus under a restrictive license. Tenable had some trouble with people rebranding Nessus as their own product, and unfortunately had to make this move. As a result, OpenVAS appeared.
OpenVAS resides at http://www.openvas.org/; the project suffered an infrastructure problem as soon as it opened, but the mailing lists show healthy development activity and the project may very well take off.