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Arcticmaniac, all of the LM78xx chips are voltage regulators, i.e. they regulate the input voltage, that must be HIGHER than the output voltage, to what ever the last two numbers are.
Next thing, when you say you want a USB charger what do you actually mean? If it is something that you plug into a computer USB socket then it will never work because the current available is too small. If, on the other hand, you want something that plugs into mains power then start with a 12v unit and add the 5v regulator and a USB socket.
When I get home tonight (midday here now) I'll take some pictures to show you.
If you added a concentric socket you could make an extremely small (10 mm x 25 mm x 5 mm) unit that you plug the 12v plug into and then plug the USB cable into for charging your USB charged devices.
12v mains charger ----> 5v regulator module ----> USB device.
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