If you are running Chrome 2.0+ or FireFox 3.5+ you will notice a dot in the upper left corner of Google Maps that should, theoretically, be able to locate your position using the W3C Geolocation API.
We have been around this block before with apps like Geode and Google Gears, but it has yet to catch on like it has with cellphones. Of course, all of this will change as more location-specific services are implemented. At any rate, give it a shot and let us know how accurate it is. It worked pretty well for me. [TechCrunch]