Bebe,
I have to say that I downloaded the Ghost Radar application. And actually several other applications as well. Being a tech guy I started to question how it worked and if the phones it was running on really had the sensitivity needed. Here's the result.
First off, the usefulness or the Ghost Radar and like applications is greatly diminished when you have your phones radio turned on. So unless you have the phone turned to "Airplane" mode, you will likely be getting tons of false triggers of EMF from the antenna itself. Same is true with WiFi or Bluetooth radios on the phone. If there are ANY forms of EMF then the entire premise of the Ghost Radar is rendered null and void.
Having said that, if you turn all those off then its fairly good. Many newer phones have a built in 3 axis hall effect sensor which detects magnetic energy. They however mostly detect magnetic lines, but not so much electromagnetic fluctuations. Also there is not static sensor. The 3 axis accelerometer can however be used for vibration sensing. All in all, its good for basics, but I would still get additional sensing gear. But a phone can go a long way towards investigations if other gear is not present. Can even be used as an EVP recorder.
I'm cheap, and I'm always a proponent of using what you have.