How do you target contacts geographically, locally?

How do you target your contacts geographically, locally?  It's pretty easy to target by country or by state / province.  But when you need to ve more precise than that, what do you do?  Just one example -- if you wanted to reach people for a seminar in Philadelphia, PA (or Paris, France -- whereever), how do you do that?

Some thoughts:

• do you use zip/postal code?  or area code? or something else?
• do you use specific Sales territories as defined in your CRM?
• do you use a third party tool to define the territory?  Mappoint? Geobatch? Tableau? Other?