Among the things that have always creeped people out are all kinds of ghosts and zombies, for one simple reason: since they are already dead, you can't kill them.
Along that line, here's what creeps me out: Xen virtual machines.
See what Wiki says about them:
Administrators can "live migrate" Xen virtual machines between physical hosts across a LAN without loss of availability. During this procedure, the LAN iteratively copies the memory of the virtual machine to the destination without stopping its execution. The process requires a stoppage of around 60–300 ms to perform final synchronization before the virtual machine begins executing at its final destination, providing an illusion of seamless migration.Imagine that. On a traditional machine, if it was doing something you did not want it to do, you would slam the keyboard. On a VM, you'd kill the monitor. Then you'd pull the host's plug. Something would work. But here, as you try to kill the guest, it floats through the air to another host. There's no stopping it. There's nothing you can do but watch it do its evil.
Next, the robots revolt against us and it's Battlestar Galactica all over again. Brrr.