Earthcall is an open-source Planet Ring server written for use in multiple Operating Systems. It is developed using the SDL.net library, so it should work fine in Windows, Unix and Mac Os X. It has been mostly tested on FreeBSD, but also runs successfully on Ubuntu and Windows XP/7/8. It should run on any Unix environment as well.
Please read and understand the AGPLv3 license if you intend to use earthcall, or run it on a server. Using earthcall on a server absolutely does mean that you MUST...