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However, what they do with the sensor information is a bit questionable and can smartphone sensors detect ghost is anyones guess.
That said smartphone really do come chock full of useful sensors which can be used in ghost hunting. They generally wont be anywhere near as sensitive as a standalone device but they can still be very handy to use.
So here is a DIY webpage I wrote that uses the android Tasker app {currently $2.99} to access your smartphones EMF sensor and produce an alert when it reaches a level specified by you ... Tasker EMF Alert Notifier.
The EMF detector in a cellphone is really just a compass chip and it really does detect magnetic fields quite well and you can set the levels you want an action to "trigger" at ...
What's really important to note is that the EMF Alert Notifier is just one of the simplest examples I could come up with. Instead of an alert you can have the action after the trigger be something else like calling you, emailing you, taking a picture, turning on an audio recording app like that Memo App I saw in a thread earlier. I've even had it run the LapseIT app to take a series of time lapse photos.
So far this has just been for the EMF sensor, BUT depending on your phones tech worthiness there will likely be proximity sensors, vibration sensors {which by the way are extremely sensitive} and on many there are even temperature and RF sensors ... all of which can be accessed and used as triggers.
Honestly you are only limited by your phone and imagination
I appologize for this being a little big BUT below is a real life screen capture of when my phone popped up an alert {similar to a notice}.
In addition to the field strength it's also quite easy to add GPS information
Now the reason I wanted to post the Tasker EMF Alert Notifier example is I made my phone use a simple chime that I can recognize. The idea is that on an investigation you will likely have your phone and if you are not working an EMF meter the chime will let you know that perchance something is there.
That said keep in mind, it takes a little time to work out what magnetic field levels are appropriate for your phone. It's basically a digital compass so you want to set the levels high enough to avoid going off near wall sockets or large metal objects.
Lastly Tasker is an App that runs continuously and some people worry that it will chew up battery life ... it doesn't
In fact even if you don't care about the above stuff Tasker can Greatly IMPROVE your battery life I use it to turn off or switch things I need or do not need based on location and time. So my wifi connection automagically turns off when I leave my home and turns on again when I get to work or a few other places I know have good wifi. There are a bazillion Tasker tricks to extend battery life so running it means your phone can run longer AND you can have any number of cool/useful "tasks" running If by any chance someone is using AutomateIT Pro ... that also works, the process is different but it still pretty easy. I run both AutomateIT and Tasker
I know many people don't have or like smartphones and that's fine. If you do have one it's basically a hand held computer packed with sensors that can call people ... so why not use it
Also old android phones are cheap and you only need the wifi connection so no carrier contract is needed FYI: an off topic use for Tasker ... you can set it up so if you send your phone a special text, Tasker will automagically call a number. This is handy for if your phone is stolen and you want the thief to call the cops and have the police go to their location 8) I haven't needed this task yet but it's a nice option to have.
I'll have what he is having 
and try to loose the phone; and there is plenty of room in the pod.... shuffling over 
