My EMF detector made it onto the Make Blog, it was a very nice surprise. This page contains the code for the project.
You can also decrease the sensitivity of the device by making a loop with the antenna, like in the photo above, or by just shortening the length of the antenna. Mess around with the configuration and have some fun. I should have also pointed this out earlier. If you are messing around with this project you should do so on a laptop not plugged into a wall outlet. Plugging the laptop into a wall outlet essentially makes your laptop an antenna for electromagnetic noise and your Arduno board will return stochastic results.
I have been getting requests for code that will average the readings; I intentionally did not include it so people would make their own and learn more. Some people have shown me some great code, but others still need a bit of help. The following link will tell you how it is done, remember though this is only one technique and there countless others out there, EMF Detector Averaging Code. This link is to an averaging program written by reader Zac Bohon.
I changed the code a bit, added an LCD, and increased the antenna size of my old EMF detector. It's sensitive enough to detect the different speed settings in my ceiling fan. The numbers are unitless, thus are are only relative to themselves. I hope one of these days I can get my hands on a good digital EMF detector so I can calibrate mine and make something more useful. I'll be posting more code and such for the project later this week.