Identify the toilet tank in the basement that no one uses that's leaking.
Squirt WD-40 on the stuck valve on the supply line to the tank.
Use elbow grease and a vise-grip to turn the supply line valve to "off".
Remind people not to use the toilet.
Check your daily usage to see that instead of using 200 units of water on a slow day, you're using 50 (!).
I don't want to guess how much water (and money) was leaking down the drain, but I think I have this problem fixed for now. The next step is to replace or repair the toilet, but until then, at least it won't be a source for high water bills.
Kudos to the City of Ann Arbor for having detailed daily water billing information online. It would only be better if they published average daily usage numbers so that you could benchmark yourself against other citizens, or if you could set alerts or alarms on your usage so that if it went over a certain amount you could get notified to see what was wrong.