The Blip service is a fast JSON API that gives you meta information about the client that you make the request from, including geolocation and browser identification (coming soon).
Here's what the response looks like when called right now, from your browser:
We make no guarantees about the accuracy of the geolocation, but we think it's precise enough to be quite useful.
Blip is completely free. We run the service as a convenience to ourselves and to others. It's also open source under the MIT License, so you're free to deploy your own Blip service if you don't want to rely on ours.