Remmina is a remote desktop client written in GTK+, aiming to be useful for system administrators and travellers, who need to work with lots of remote computers in front of either large monitors or tiny netbooks.Remmina supports multiple network protocols in an integrated and consistant user interface. Currently RDP, VNC, XDMCP and SSH are supported.
Remmina is free and open-source software, released under GNU GPL license.Currently,the latest version Remmina 0.7.4 released on February 18, 2010.
Install in Ubuntu

Add Remmina PPA by editing /etc/apt/sources.list with this command (or open system->Administration->Software Sources and add Remmina PPA source.)

sudo gedit /etc/apt/sources.list

append this line into the end of the file:
for 9.10:

deb http://ppa.launchpad.net/llyzs/ppa/ubuntu karmic main

for 9.04:

deb http://ppa.launchpad.net/llyzs/ppa/ubuntu jaunty main

Add the key and update:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 5A0FA8F1
sudo apt-get update

Now,there should be three packages in ubuntu repository:
* “remmina”, the main GTK+ application
* “remmina-gnome”, the GNOME desktop applet
* “remmina-xfce”, the Xfce panel plugin
Install them from synaptic package manager,or use the sudo apt-get install packagename command to install them.

Remmina mainpage:http://remmina.sourceforge.net/index.shtml

Screenshots (from Remmina mainpage)
remote-desktop-client
remote-desktop-client1
remote-desktop-client2
remote-desktop-client3