Here's one for the extreme system builders out there. Putting two systems inside one PC case would normally require two power supplies, but Phanteks' Power Splitter accessory neatly solves that problem.
This device takes power from one 24-pin ATX power cable and two eight-pin auxiliary connectors, splitting it between two sets of outputs with 24-pin, eight-pin, and two four-pin connectors. Phanteks says that each system on the splitter can be turned on or off independently of the other—so long as one PC is running, the PSU will also remain on.
The splitter measures 6.5" by 1.2" by 3.9" (WHD), and it can safely provide a maximum 336W of 12V power, 70W of 5V, and 46W of 3.3V juice across both systems. Graphics card power is excluded from those calculations. If you need this splitter, we're betting you have a PSU with plenty of PCIe auxiliary connectors, anyway.