Which running/biking apps (Strava, Runtastic, Endomondo, ...) use barometer for proper elevation gain?
iSmoothRun records elevation data using the iPhone's barometric altimeter. This app can sync with various storage and tracking services, such as Dropbox, RunKeeper, Strava, etc.

The barometric data is preserved when syncing or exporting GPX/TCX files from iSmoothRun (although I currently have to tweak the header of the GPX file and manually upload to Strava in order for Strava to respect the barometric data recorded by iSmoothRun instead of discarding it in favor of its own measurements based on topographic data).