If your hose bib (otherwise known as the garden faucet outside your house) is leaking, it could be that the Air Vent Assembly is defective, or just worn out. If this is the problem, you'll know because the water will be coming out from under that little green cap at the top of the faucet.
Turns out they are super simple and cheap to fix. McClendon Hardware in Woodinville carries replacement parts. Just unscrew the old one and screw the new one in. You will just need a set of pliers.
No need to turn off the water main. Just take off the hose and make sure the faucet is off. Cost: about $9.00.
Here's what it looks like: