"VNC stands for Virtual Network Computing. It is, in essence, a remote display system which allows you to view a computing 'desktop' environment not only on the machine where it is running, but from anywhere on the Internet and from a wide variety of machine architectures."
VNC is absolutely free and works under many different platforms including Win32, MacOS, various Unix-like systems including Linux etc.
See also: TightVNC is an improved version of VNC.
See also: VNCScan Console http://www.vncscan.com - The original and most stable front end for all flavors of VNC.