For quite a while I have been an enthusiastic fan of VNC (better: tightvnc) when it comes to access a remote server including graphics. However, since the VNC variant used in my Laptop OS Ubuntu Dapper (and therefore the OS of the PCs of my wife, father, mother... :-) ) is quite slow, I was looking for alternatives.
I found one with the NX products from http://www.nomachine.com which allow me not to just start a graphical session on a remote machine (via a possibly slow line), but also attach to an existing session on the remote machine (even the graphical root window) or disconnect/reconnect to a session. There are free variants for Linux and Solaris available, unfortunately not for Windows, but VNC server or rdesktop (RDP) are ok in that case.
The most amazing experience for me was the installation. Klick on the download button and let your package manager (the debian one in my case) do the rest. Since the complete login and traffic works through ssh, no extra tunnel in a firewall etc. must be opened. An existing ssh based access infrastructure can be used.
There is also a fully GPL-ed variant called FreeNX which lacks some features like reconnecting etc.