OpenNAP is free software which replicates the functionality of a Napster server using the Napster protocol. Because this software is released under the GNU GPL, it can be used and modified freely by anyone for any purpose.
OpenNAP runs under Windows NT and Windows 2000, which means that it can be run on a massive installed base of computers, as opposed to the relatively smaller base of Linux and Unix 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.