Dating websites don't have anything on you now. Convert an Internet user's current IP address (like 184.108.40.206) to a city name, or even geo coordinates with these IP lookup APIs. And do it for free.
Hostip.info is a community-powered database of IP mapping. Its REST API is easy to incorporate into server-side code, with several options for the type of output. Check out our hostip.info API profile, where you can see the mashups already using this API.
Blogama (now IPInfoDB) is a new IP geolocation API with streamlined data based on the MaxMind API. Marc-Andre Caron reduced duplication and surprisingly has a database of only 1.2 million records that is still incredibly accurate to the city level. Even better, as Make pointed out, you can load the data into your own database for speedier access. Our IPInfoDB API profile.
MaxMind is an API of a different sort. Rather than call to a web service, its free version is distributed as a binary. There are open source libraries for common programming languages to access the IP data. Find out more in our MaxMind GeoIP API profile.
Now that you can find your user's location, what can you do? For starters, how about centering your map mashup automatically on the user's city? If your site requires registration, why not help fill out that profile?
How will you use this geo data? Is it cool, or a little creepy?