avAPRSSrvr is a package of APRS server applications written in Java. It will run on any platform that supports Java. It includes a variety of server functions: digipeating, igate, email, filtering, etc. .
javAPRSSrvr is not open source but it is available for free on request.
Recent releases of javAPRSSrvr support the NSR No Source Routing digipeating algorithm.
I believe that most if not all of the first and second tier APRS-IS internet servers are running some form of javAPRSSrvr.