My main water supply valve is leaking through the stem packing. Is it possible to repair the leak by shutting off the valve and then replacing the packing? I don't want to have the water shut off at the street for obvious reasons.
Most valves you can replace or add to the packing wile it shut off without a problem. Sometimes you can just tighten the packing nut a little to stop a leak. You might find this article useful. It has some nice diagrams of valves. Note: most main shut off valves I have seen are the gate valve style.