A month or so ago, hydrobond did me the favor of giving me this great little program. It can be used for some GREAT practical uses, as well as just freaking the heck out of unsuspecting family members and friends.
Basically, it's a remote desktop program, but what's great about it is that it can be set up so that if a person is on one networked PC, it won't log them off when you log on. What this means is, they'll just be minding their own business, using their PC, and you take control - without their knowing, of course. BAM, their mouse moves.... or BAM, their programs start closing randomly... or BAM, they start saying things to friends on AIM like "I eat poopy," not having a clue what hit them. All they can do is watch in horror as their computer seems to take over by itself. This works especially well on family members who aren't very tech-savvy and have never even heard of remote desktop access.
And, of course if you don't want to be mean to family members, it still has lots of practical uses. You can basically have all of your home PCs accessible from one monitor, without even setting up a KVM switch or even having them in the same room. And, you can still use the program from anywhere that's hooked to the internet, assuming you have everything set up right.
I didn't download the program from here, but I think this is the same thing. Enjoy:
http://www.realvnc.com/download.html