This page demonstrates Geo IP service from WikiMedia. How to get geo information by IP address (client side solution):
1) add the following code to your page: <script type="text/javascript" src="http://geoiplookup.wikimedia.org"></script>
2) analyze JavaScript object variable Geo after loading your page. Attributes are: Geo.city, Geo.country, Geo.lat, Geo.lon
Actually, you can replicate this service with MaxMind database for example
Related mashups:
Geo traceroute Geo IP taglib All mashups