is a snap.

Just check out the FreeNX page on the Ubuntu Wiki

What I did…

$ sudo cat >> /etc/apt/sources.list
deb http://free.linux.hp.com/~brett/seveas/freenx/ breezy-seveas freenx
$ gpg --keyserver subkeys.pgp.net --recv-keys 1135D466
$ gpg --export --armor 1135D466 | sudo apt-key add -
$ sudo apt-get update
$ sudo apt-get install freenx nxviewer
$ dpkg --list | grep NX
ii  freenx                                 0.4.4+0.4.5-3ubuntu0               The FreeNX application/thin-client server ba
ii  libxcomp1                              1.4.92+1.5.0-4ubuntu0              NoMachine NX - NX compression library
ii  libxcompext1                           1.4.92+1.5.0-4ubuntu0              NoMachine NX - NX compression library
ii  nxagent                                1.4.92+1.5.0-4ubuntu0              NoMachine NX - nesting X server with roundtr
ii  nxclient                               1.5.0-135                          NoMachine NX Client and Libraries.
ii  nxlibs                                 1.4.92+1.5.0-4ubuntu0              NoMachine NX - common agent libraries
ii  nxviewer                               1.4.92+1.5.0-4ubuntu0              NoMachine NX - nesting X server with roundtr