NoMachine (the free version) lets you access the physical desktop remotely and requires that the Xserver is running. You can also use NoMachine on a headless Linux server (without the need of having the X server running), more instructions are available here:
https://www.nomachine.com/AR10K00710
Any of the products from the Terminal Server range (e.g. Workstation or Terminal Server, https://www.nomachine.com/enterprise), allows to create virtual desktop sessions. In this case it’s not necessary to have the Xserver running.
The free 3, NX Free Edition, worked as a terminal server limited to 2 users. This functionality is no longer available in the free version.