There are various places in the home where water valves are located. One prime place is where the water supply enters the home. This is in place to shut off the supply for maintenance. But over the course of time, these can develop a leak. Not only does this waste water but overly wet soil at one corner of the home can cause the slab to crack. It happened to a friend of mine.

But before a costly plumber visit, why not try to fix it yourself? Valves are pretty simple so there's really a few fixes. First, try to tighten the packing nut (located just beyond the handle). If the handle is too hard to turn, the packing nut is too tight.

That didn't fix it? You'll need to shut off the water upstream. This is usually at the water meter. Next, remove the valve handle and the packing nut. Replace the packing material and put it all back together. Turn the water back on and you're back in business!