Is there a 3rd-party app that uses iPhone 6's barometer? I have an iPhone 6 and was wondering if there is any app that will simply display the current pressure using the barometer built into the phone?

Or an app, other then the built in HealthKit which tracks stairs climbed, that uses that barometric pressure for something?
There are quite a few Apps in the App Store, including Barometer, Barometer+ and Barometer++. Take your pick. This site does the research for you.