The JackPC is a complete PC that can be directly embedded in the wall.
While this may make upgrade a bit trickier than usual, it's kind of cute.
It's been around for a while, but I just saw it mentioned again, on OhGizmo!.
I have to admit, I can't see a lot of value in it, since you'd need to plug a monitor, keyboard and mouse in to get any use out of it. It's just clever.