95% of the time I'm dealing with Michigan regarding county line problems.  Finding a phone number for any assessor -- and then actually getting an assessor to call you back -- is just about impossible and/or takes days I don't have.  (Am I right fellow Michiganders??)  Being able to to quickly get on line and find the right county within seconds is imperative.  We have one village here - Cement City - that can be in Jackson, Hillsdale or Lenawee.  Plugging in an address and seeing which side of which county line a particular place falls can help me keep an order.  If I have to delay an order looking for more info, clients sometimes cancel (and -- I'm assuming -- go elsewhere).

Using Bing maps as Anita suggested came in handy once already. 

What we need is an IT guy who can come up with a mapping solution that's quick and clean!