Controlling a Mac remotely

Solution 1:

The Mac comes with the server part of the software built-in for "Apple Remote Desktop"; They can be viewed and managed remotely if you have the client software. For most of us that's too expensive a solution, so an alternate is to use VNC. Check out JollysFastVNC and Chicken of the VNC.

Also, iChat, which comes with the Macs, can do screen sharing, which might be all you need. Check out "Share and share alike." from Apple's iChat site and "Sharing your screen with a buddy".

Solution 2:

There are a lot! LogMeIn is one of them, which has the benefit to have a web interface and doesn't need your parents computer to have a fixed IP. You install the client on their computer, register it and then you'll be able to control it via your interface.

Give it a look, it's quite good. The free version doesn't have all advanced features (obviously) but it's quite enough for remote help of parents' computers (I use it for this same purpose and I also do it for computers in Canada while I'm in Europe!)