Taking control of a remote desktop

[skinnylegs]

I thought Macs had software pre-installed or functionality built-in that allows users to take control of a remote desktop? If so, where is it?

My sister bought a new Mac and I was 'gonna show her a few things so that's why I'm asking.

 

[BSchorr]

You can do it threw iChat. But you both have to have a Leopard computer.