Last week I was able to check out iChat's new Screen Saving feature that allows me to see and even take over someone else's desktop (who also must be running Leopard).
This was really helpful in troubleshooting an error in seconds and explaining it while it was being fixed, instead of trying to explain everything over the phone.
One cool feature is that it scales the other person's screen so it fits nicely on your own. This allowed me to work with a 24" iMac on my 15.4" MacBook Pro.
There are just a couple of caveats - you have to have good upload speeds for smooth operation otherwise the response can be a little choppy. Setting your background to black instead of your favorite abtract rainbow spectrum might help with that too.
The other caveat is that if you're controlling someone else's screen, the moment he or she touches the mouse, you lose control until the mouse on the other end stops moving. So tell the person to sit on their hands or pick up that Rubik's cube that they have never solved.