I've been using a Function Generator application on my phone that has saved me from going out and buying the same. Granted, I am limited to the human audible range (well above my hearing level), but it is just another example of how applications have done so much damage to the market for actual hardware.
I'd like to have an application (maybe for the tricorder?) that alerts me when I've been offered a bunch of crap instead of answers. Lies ought to be most painful to the lier! Perhaps we'd call it the con artist detector. It would be very popular!
A credibility checker might also be a good application. Might include a background search on the person one is talking to. How about some explosives inside the phone just for those who want to go out with a bang! Perhaps with a trigger tied to the arming button? Could be interesting! Maybe we will be able to download a smell or taste one day! Wouldn't that be cool! What kind of apps would you like to see?