There is only one Remote Desktop server for Linux/Unix: xrdp. It's an open source server based on the rdesktop client. xrdp renders an X window desktop rather than a Windows desktop, but is still compatible with rdesktop and the standard Microsoft Terminal Services/Remote Desktop clients.
If you're looking for clients, go to the Remote Desktop Clients index.
If you want to check out servers for a different OS, go back to the Remote Desktop Servers index.