The Cellphone Spy Recon from Brickhouse Security is designed to help you keep an eye on your kids, stalk your significant other, or spy on your employees, but using it is a bit complicated. First, you'll need to get your target's phone and install the spying application. Then, you need to download an app onto their phone. After the app is installed, you'll need to grab the device's Mobile Equipment ID number (MEID) (which you can find by dialing *#06#) and jot it down.
Then you take the included USB drive and plug it into your PC. The USB drive will automatically take you to the Cell Phone Recon Spy website where you can register for an account and insert the MEID of the phone you wish to spy on. You can't read all the emails or text messages on the device, only the ones that have been sent or have arrived after you installed the spying software. You can also view call logs, but cannot listen in on the actual conversation.
At $169 per spying software license, however, it might just be cheaper, and probably less complicated, to follow them in person.