An increasing number of appliances draw a very small current from the power supply. If you need to design a mains-powered device, you could generally choose between a linear and a switch-mode power ...

5V PCI slots have the "key" far away from the back of the PC case.. If it was a 3.3V slot, it would be keyed with the key much closer to the back. You don't typically see 3.3V slots in desktop boards, ...