What services can I used to find a user's location based on their IP address?

Question

If you need to locate a user based on their IP address, what services are available (free and not free services are fine)?

P.S. I understand that some users use prox

Answer 1

There was a brilliant solution using those adult ads ie. "women in YOUR_TOWN would like to 'meet' you tonight"

Somebody worked out a way to extract the location string without loading the ad.
I can't find the code, searching google for adult ads isn't terribly effective, but somebody here might remember it.