Google have released two new ways to find a web user's location without requiring any user input.
1. The AJAX API Property (dead link removed)
The AJAX API Property retrieves an approximate location from a user's IP address. Google Blogoscoped has released an example Google Map that uses the AJAX API property to centre the map on the user's location and provide the results of an image search based on the user's location.
2. The Gears Geolocation API (service now discontinued)
The Gears Geolocation API provides a way to get a more precise estimate of a user's location. On mobile devices with Gears installed, the Geolocation API can triangulate a location on nearby cell-phone masts to get a more accurate position fix.