Geosense is a Windows sensor which supports the Windows 7 Location and Sensors platform.
Or, in other words, if you have a location-aware application - a suitable Twitter client, say - which also makes use of Windows 7 sensors, then installing Geosense may allow this to retrieve and use your current location, even if your system doesn't have GPS hardware.
How? The authors explain: "Geosense is designed to use a hybrid mix of geolocation service providers and geolocation methods to pinpoint the most accurate location information possible - including but not limited to WiFi triangulation, cell tower triangulation and IP lookup."
The results are far from reliable, though. In our tests Geosense was 150 miles out on one system, and reported that we were in the middle of the Atlantic on another. Only one of our three test systems managed to return a reasonably accurate position.
The program is free of charge, though, so there's no harm in giving it a try. And even if you don't have any location-aware apps, the authors have created a simple Google Maps client which you may find useful. Set it to AutoRefresh on a laptop, for instance, and your current position will be regularly updated on a map. (Or some other apparently random position, perhaps - try it and see.)