What can I do with the barometer in my phone?

I don't know about novel uses beyond the obvious, but maybe you're not aware why it was put in there? It turns out that using the pressure to make a rough estimate of the altitude allows the GPS signal to lock much faster than otherwise. That was why it was put in, in the first place... and on mine, I do notice the GPS locks a lot faster than my previous phones.

(Here's a https://plus.google.com/112413860260589530492/posts/jVJhPyouWDP.)

You could put the phone in a bell jar with a vacuum pump attached, and the barometer could tell you how near you are to creating a vacuum. Regarding gilrain's explanation and link - I'm surprised a barometer would make much difference for locking on GPS, especially using a service like Google Maps, that should be able to give a more accurate estimate of altitude based on approximate latitude and longitude, than a barometer could provide, in most cases. I guess it puts less strain on the mapping service.

I'm pretty certain when my elderly, rheumatoid arthritis racked ancestor complained about their pain, and that they can "feel" the wind (i.e. like the bends, feels worse when the pressure falls), I ought to be able to judge what kind of a day they're having... except it doesn't quite work like that. I usually have to make do with a sealed plastic fizzy-drink bottle. I open it each day, and if the air rushes out, today's lower than yesterday. If the air rushes in, it's higher. I also estimate by how long my cafetiere takes to boil in the morning. 5 mins is the norm.

Go on a hike. Measure your change in altitude over distance. Then you can see what the total amount you climbed was (because you usually climb much more than the total difference in altitude between trailhead and peak, due to the many rises and falls in between). You'll also be able to make a little chart of the path you took. Which makes me wonder if you could use the barometer in conjunction with your GPS and make a 3D drawing by moving the phone around--perhaps by swinging it on a string from a building or climbing through trees, or something clever and fun. Maybe you could take the phone and touch it to every square inch of some great monolith, and have a 3D drawing of it.