I'll bet you live in an older house. After years of turning a faucet off and on, the washer that holds the water in hardens or wears down. This can be repaired if you're fairly handy with tools. Turn off the main supply as much as possible. If the flow is not too great, remove the screw holding the knob on and pull it off. Use a wrench to remove the packing nut. Put the handle back on and turn it counter-clockwise till the entire stem comes out. Take it to a hardware or home improvement store and ask them for a new washer and packing. Put it all back together the way it was removed. Fixing the main supply may be more difficult depending on where your shut-off is. In some cases, you may have to call the city to turn it off so a repair can be made. This should stop the leak IF the seat is not corroded.