Nvidia's Optimus lets you pull a GPU out of a running system
 

Apparently, you can pull an Optimus-capable discrete graphics module out of a running notebook, then put it back and run a graphically intensive app, all without a hitch. That's because the system completely powers down the GPU module (and the associated PCI Express interface) when it's not needed, relying on the system's integrated graphics, instead.

http://techreport.com/discussions.x/18554