Google Maps for Mobile users have had access to a feature called My Location that does exactly what you would think: marks your current location. Now, the desktop version of Google Maps has access to that feature.
You've got to be using a compatible browser. So, if you haven't upgraded to Firefox 3.5 or Chrome 2.0 (or any browser installed with the current version of Google Gears), then it's time to get your download on.
Then when you load Google Maps, look for a teeny tiny new box between the zoom in/out bar and the compass circle. Click that box and you'll be prompted to give permission to share your location.
My Location seemed to figure out that I was in the greater metropolitan area of Raleigh, NC, but there was no indication via the famous blue circle that the feature had worked to find my general location. It certainly couldn't pinpoint the location of the public library whose Wifi I'm currently schlepping. Google uses nearby Wifi points to determine location, but the LatLong blog did say it may not be able to provide one every time. Though, I'm always flabbergasted when new tech like this doesn't work in such a high-tech area as the Raleigh-Durham area.
Were you able to get a more precise location detection from Google Maps? Do tell in the comments.