Since X-windows is bundled with MacOS X versions 10.3 (Panther) and above, your best bet would be to install same if you're running this version of MacOS X.
Once you have done so, launch the Macintosh X11 application by double-clicking. You should be presented with a X-based shell window. Within that window, enter the command
An older version of VNC is available for Mac users who are using MacOS 9.x and below, at http://www.cl.cam.ac.uk/research/dtg/attarchive/vnc/macvncviewer.html.