Maidenhead Grid, courtesy of geolocation in HTML5
If you are using Firefox, the accuracy might be good, or might be bad. It really depends on how effective Google’s IP based geolocation is in your area. At my brother’s house in Hillsboro, OR (near the Intel Campus) it was very, very accurate. Of course, if you are using the iPhone, it uses the built-in GPS. Feel free to swipe this idea and use it however you see fit.
If you find this useful, please let me know, and I’ll consider making a more permanent home for it. I’d also welcome any other suggestions on things that might make it more useful.
Addendum: I made some improvements (notably, to also give the accuracy of the measurement, and added a small google maps display) and IRC #hamradio buddy autojack confirmed that it works on his Android phone, and thoughtfully provided a couple of screenshots.