Using the Google Maps API, you can do this pretty easily. Just add a click event handler to your map object, which passes in the latitude/longitude coordinates of where the user clicked (see details on the click event here).

function initMap()
{
    // do map object creation and initialization here
    // ...

    // add a click event handler to the map object
    GEvent.addListener(map, "click", function(overlay, latLng)
    {
        // display the lat/lng in your form s lat/lng fields
        document.getElementById("latFld").value = latLng.lat();
        document.getElementById("lngFld").value = latLng.lng();
    });
}

The code is for google map api version 3. v2 has been depreciated so better to stick with v3.

Have a div with id mapdiv in which the map is loaded

Use an input element with id latlng to display the lat and lng when you click on the map. so that people can copy that value into another application. you can modify the opts variable according to your need.

Add an event handler for click event like in the code. So that when you click on the map the input element is populated with lat and lng values.

var map;
function mapa()
{
    var opts = { center : new google.maps.LatLng(26.12295, -80.17122),  zoom :11,  mapTypeId : google.maps.MapTypeId.ROADMAP }
    map = new google.maps.Map(document.getElementById( mapdiv ),opts);

    google.maps.event.addListener(map, click ,function(event) {
        document.getElementById( latlng ).value = event.latLng.lat() +  ,   + event.latLng.lng()
    })      
}

The above is all that you need.

if you want to see the values of lat and lng when you move the mouse over the map then add a mouse move event listener like in the following code. you can use a span or even an input element to see the values. I had used span in my code.

google.maps.event.addListener(map, mousemove ,function(event) {
    document.getElementById( latspan ).innerHTML = event.latLng.lat()
    document.getElementById( lngspan ).innerHTML = event.latLng.lng()
});

You will need to know the lat/long of at least TWO points on the map, preferably not very close together, and also know their pixel coordinates relative to one corner (i.e. the best two points would be opposite corners). Then it s simple math to convert the pixel coordinate of the click into lat/long by interpolation. However, note that this works only as long as the area covered by the map is "flat" relative to the Earth s curvature. If the map covered a large area, say 6000 miles wide, there would be significant distortion. If you needed this to be accurate you d have to do some map projection to calculate the position.

Check out the Google Maps API as well as this link: http://www.gorissen.info/Pierre/maps/googleMapLocation.php

I think that the Google Maps API probably can do what you re looking for.