Virtual Network Computing, or VNC is a free cross-platform tool for using remote computers, similar to "PC Anywhere". It uses a standard IP connection, some versions can also use ssh for security. You can use the remote computer just like your own. It works fine with all kinds of X displays, all current Windows versions, ...
You can even use any Java-enabled Web browser as a client, because any VNC server [!? CL certainly thinks he's encountered VNC servers without Java awareness] will automatically send the necessary Java applet to the browser.
On Windows you become a second user of the graphical display - thus interfering with the person sitting at the computer, while the server version on Unix creates additional, invisible X displays which are only visible on the remote side. So on Unix computers several VNC servers can run at the same time without interfering with any other session on the same machine.
RealVNC has nice enhancements, including automatic configuraton that tunes the encodings to suit the available bandwith. [KBK: "I found that the current RealVNC is incompatible with the 'vncserver' that comes with SuSE 9.2 - so switched to TightVNC, which seems to work better.""]
CLN 2005-02-14: Can anyone verify the rumor that VNC violates the 'Windows XP license agreement? Specifically, I've heard that the XP EULA disallows non-Microsoft remote viewers for the XP desktop. A stupid and petty restriction, IMHO, but, hey it's their s/w, they can license it however they want.
XP bandwidth brouhaha ,Brian Livingston, supports the rumor you heard.
George Peter Staplin 2008.05.21 - NexTk has a VNC widget. It supports the mouse and keyboard. It needs a bit of code for some keysyms, such as function keys, page up, page down (in ntk). It only supports true-colour displays. If anyone is curious the code is here: http://megapkg.googlecode.com/svn/trunk/NexTk_objstructure.vfs/vnc.img It could use support for authentication protocols. I could use some help with it. I used this protocol document to implement my VNC widget: http://www.realvnc.com/docs/rfbproto.pdf
SEH 2008-08-25: ssvnc is a vnc client which makes secure vnc sessions easier by automating ssl/ssh tunneling as the vnc connection is established. It includes a simple Tk GUI for specifying connection settings: