OpenNAP runs under Unix-like Operating systems like Linux, while also on Windows NT and Windows 2000, which means that it can be run on a massive installed base of computers. This makes it relatively likely that many people will be running OpenNAP for groups of friends, or openly on the Internet in general.
The main website for the OpenNAP project is here.
OpenNAP clients:
SlavaNap is an alternative OpenNap server.