Different ad based on geo location

I'm not sure if this is really cloaking, but I guess it's a type of ip delivery. Here's what I want to do. There are many sites, search engines, ads that deliver different contents based on your location. Similarly, I want to show different ads based on the visitor's city since it's a local ad. Any idea how it's done, or can anybody point me to a right direction?

It can be done by using any GeoIP database (http://dev.maxmind.com/geoip/), but you have to implement such location-based behavior of your website by yourself.

Yeah, saw that. Hoped there would be a ready-made solution. Thanks.