at:
These are very common relay drivers. They basically don;t need power because they are basically Darlington transistors when you remove the internal diodes.
Common is somewhat of a misnomer, but it's really +V or the common point of a relay that is connected to ground by the ULN2003 to turn them on. So, the device contains the internal coil suppression diodes. They can be left unconnected when driving things such as LEDs.
They are also useful because with an input in High Z, the relay is off, A low it is off and a high it is on. So when a micro powers up as an input, the relays are off.