I run an NX server and tightvncserver on my linux box (Ubuntu 10.04), with default settings. I have NX client and Chicken of VNC on my Mac (Snow Leopard).
I'd like to share desktop of the linux server with my Mac client, i.e., control the existing Linux user session from Mac.
But it seems that on my Mac both NX and VNC connections start a new session on the server. I'd love to know how to do a "desktop sharing" instead of standalone remote session.
Try winswitch - it will give you a GUI where you can start/stop sessions, the X11 display of your Linux machine will be shown as an existing session which you can "shadow"/"copy" via NX or VNC. (there are installers for macs and windows too)
If you want to do this by hand you can use x0vncserver for VNC and nxagent -S for NX, but then you will also have to deal with port configuration and IP/hostname addressing.. (as well as configuring nx DISPLAY environment variable which is totally non trivial)