In addition to OS X having a hidden VNC Client, since the release of Tiger (OS X 10.4) there is also a hidden VNC Server included. Starting the server is a simple as checking a box in the Sharing preferences. 

A VNC server allows you to control your Mac from another computers. The other computers do not need to be Macs; they can be Windows PCs.

Starting the Mac OS X VNC Server

Open System Preferences:

Select the Sharing preferences:

 Enable Remote Management:

Click on Computer Settings to get to the VNC options:

Enable VNC viewers may control screen with password. Enter a hard to guess pass phrase. You will need this to connect from the remote computer. Click on the OK button to save your settings.

NOTE: In the Sharing pane you see a section that Allows access for either all users or those identified. This setting is not for VNC access but controls Apple’s Remote Desktop applications access. ARD sits on top of VNC and gives System Administrators full access to control and manage Macs remotely.

Close the System Preferences window.

You are now running a VNC server and can connect remotely to your Mac.

To connect you need a VNC Client. If your remote computer is a Mac running OS X 10.5.x, you can use the build-in hidden VNC Client. See this postinghow to activate the client.