A VNC client lets you connect to a desktop that's been shared on another computer. Excellent VNC clients are available for every major Linux distribution and other operating system.
List of clients
krdc is the standard KDE client, and can also act as a Windows Terminal Services client (click here to install krdc)
xvnc4viewer (RealVNC viewer) is a simplistic client recommended for Xubuntu users
xvnc4viewer is available in the Universe repository (click here to install xvnc4viewer)
xtightvncviewer (TightVNC viewer) is a simplistic client recommended for use with tightvncserver
xtightvncviewer is available in the Universe repository (click here to install xtightvncviewer)
RealVNC has a Windows version available
TightVNC also has a Windows version available
RealVNC Java client is a multiplatform Java client that can run in a web browser
TightVNC Java client is also a multiplatform Java client that can run in a web browser
If you can't install software on the machine you'll log in from, you can make this available through a web server. It's available in the vnc-java package in the Multiverse repository
Connecting to a VNC Server
To connect a VNC client to a VNC server, you need to know the name or the IP address of the computer you'll connect to, and the port or display number of its VNC server. By convention, port numbers start at 5,900 and go up, so a computer that shared three different desktops would normally listen on ports 5,900, 5,901 and 5,902. Display numbers use the 5,900 convention to try to make things easier - port number 5,900 is display number :0, port number 5,901 is display number :1, and so on.
If you accidentally display the remote desktop fullscreen, try pressing F8 or F11 to exit.
If you connect to the VNC server but can't change anything, it's possible the server or client is in view-only mode. Both server and client need to be configured to let you control the keyboard and mouse.
If you connect to a VNC server, can see the initial desktop, can see the mouse moving around, but the rest of the screen doesn't update, then you probably need to disable desktop effects on the shared desktop. A known bug means that desktop effects are currently incompatible with most VNC servers.