A friend and I have been planning this for a few months. On a laptop, we can find the signal strengths of wireless access points, and use this information to determine location. A few days ago, I sat down and coded it, in time for Expo. My poster probably says it best:
Instead of using triangulation, it simply acquires a lot of data. (Triangulation did not work as well, because of the many walls and windows in our dorm.) The database simply records associations of a set of wireless signals with a name ("AC Room 318") and a point on a map. To look up a position, the server looks for a similar set of values with a distance formula, and returns the closest match. When the program is run, it recieves feedback from users, and every time adds a new association. Over time, the results get better and better. From successful tests, I think room-level precision may be possible.
We were able to figure out the right WMI query to find signal strengths other than the current connection (in Windows). I then wrote a small C# app that runs the query and prints the results to stdout.
A Python program gathers this data, taking the average of three runs, and sends it to the server. A PHP script forms a vector, thinking of each of the 76 access points as an axis, and looks for nearby vectors in the database. The location of the user, and status, is also stored in a centralized database.
Finally, in a web app you can view locations of everyone who is logged in. Hovering over the icons tells you who and where the person is. (Only if you swear that if you are up to no good).