A VNC server is included in Mac OS X 10.4+
http://www.dssw.co.uk/blog/2007/05/14/a-vnc-server-is-included-in-mac-os-x-104/
VNC® Enterprise Edition for Mac OS X
http://www.realvnc.com/products/enterprise/macosx.html
How to Connect VNC Server in MAC OSX Leopard.
http://www.ihackintosh.com/2009/04/how-to-connect-vnc-server-in-mac-osx-leopard/