Remember what snakes eat; black snakes, king snakes (typically black in color), rat snakes (like the one in the water picture a couple replies above) take care of rodents for you. Garter snakes may get a rodent or two, but help out in the bug department a lot, too. They may get a frog or toad or two, but...
I'd rather have snakes about than mice and rats. They're not aggressive (except when shedding sometimes) and I can't imagine any reason to kill them.
Now, a copperhead.. I might change my tune a bit.