I think that it is safe to say that most of the people who make (big or small) electronics-projects have a pc or laptop in theire hobbycorner and a lot of projects need 5V for IC's or microcontrollers. So using power from a USB cable isn't that farfetched and lets face it: a lot of devices around us use a USB-connection to get their power or to charge their batteries.
Step 1: About USB-connectors and -power
For this Instructable you can use any USB-connector you like (or you can even cut the cable and solder the wires) but I will use a B-type. The pins always have the same function for all types of connectors.
Voltage = 5V
max Current = 500mA